Let's assume both Akainu and Sengoku ended up dying in Whitebeard's place. Because, I think that makes things more interesting. Even if it is pretty unlikely.
Just to kick things off, let's say that the point of divergence is...oh, I don't know. Shanks arrives early to Marineford, and somehow tricks Akainu and Sengoku into punching each other so hard, they break each other's necks? (Cue the shocked reaction faces from literally everyone!) Sound like a good enough excuse? No. Too bad. Let's move on.
On the Marine's side:
Aokiji becomes the new Fleet Admiral by default, since Akainu isn't around to challenge him. Aokiji never joins the Blackbeard Pirates. He's pressured into still being pretty harsh on pirates in general, due to Akainu and Sengoku's deaths. But, he's also able to weed out some of the corruption in the Marines. The Marines are now generally more "tactical" than aggressive in their maneuvers.
Kizaru stays on as a Admiral. But, with his old friend Akainu's death, he's more focused and driven then before. He's put in charge of hunting down the Whitebeard Pirates.
The World Military Draft still goes into effect and still recruits Fujitora and Ryokugyu. But, with his idol Akainu dead, Ryokugyu is even more of a zealot than he was before. Butting heads with Kizaru over who gets to attack the Whitebeard Pirates.
If we want to assume that Aokiji in the main timeline is secretly a double agent spying on Blackbeard for SWORD, I think he'd need a replacement. And...who better for that than Garp? Garp has been publicly VERY critical of the Celestial Dragons before, so it'd be pretty believable for him to fake leaving the Marines and joining up with a different pirate crew. He might be the worst undercover agent of all time, and he might kind of hate pretending to be a pirate. But, I think he'd do it if his friend Sengoku died in front of him during Marineford. Garp would feel guilty for "failing" his friend, and be more willing to do something to take down other potential threats.
But, Garp isn't going to be joining up with Blackbeard's crew, like Aokiji did. For reasons I'll explain later. Instead, Garp is going undercover in Kaido's crew. I think they would appreciate each other's strength. Garp and Kaido actually become pretty good drinking buddies.
Koby is not informed of this. And many a hijink is had because of his confusion about this whole situation. Koby still dedicates himself to training. But, with Garp gone, it's slightly different. Koby is taken under Fujitora's wing. Where he learns more "sensory" based attacks. Koby also ends up with Sengoku's Buddha Fruit.
On the Whitebeard Pirates side:
Whitebeard doesn't die at Marineford. But, he's still a sick old man, so he's going to die, eventually.
After Marineford, Whitebeard decides it's time to deal with the guy who started all this. Whitebeard decides to bring his whole crew with him to help hunt the traitor Blackbeard down. Betraying your family once is unforgivable. Doing it TWICE?! Now it's time for father's wrath!
The Strawhats are reunited. And Luffy takes a brief pause on his journey to becoming the Pirate King to help the Whitebeard Pirates hunt down Blackbeard. Shanks also joins the fight. The White-Red-Straw Alliance is born.
The Blackbeard Pirates are overwhelmed, and several Blackbeard crew-members die. But, Blackbeard himself manages to escape when Kizaru and the Marines show up to attack everyone. Everyone assumes Blackbeard died in a big explosion.
Whitebeard dies during the fight, but he's able to say a few parting words to Ace.
Ace would then take over the Whitebeard Pirates. Ace recommits to finding the One Piece. The Whitebeard Pirates never disband.
Ace becomes one of the Yonko by default. Not Blackbeard this time around.
Luffy briefly considers joining Ace's crew, since he almost lost him...TWICE, at this point. But, Ace is actually the one to talk him out of it. "Who's going to be my rival, if you did something stupid like that? I want to beat you fair and square, little bro!"
Luffy, Shanks, and Ace all depart. Each one declaring that they'll be the one to be the Pirate King in the future.
On the Strawhats side:
Luffy didn't lose Ace. But, he's still able to tell he's weaker than he should be. He wasn't able to help out at Marineford or during the attack on Blackbeard as much as he would have liked.
The Strawhats all decide to return to where Kuma sent them to train. But, only for 1 year this time, instead of 2 years. Luffy's still a little cocky from being able to save Ace. Plus, he doesn't want Ace and Shanks to get too much of a head start. This means the Strawhats won't be as strong Post-Timeskip as they were in the Original Timeline.
On Blackbeard's side:
Blackbeard now no longer has no crew. And he still only has one Devil Fruit. AND he's hurt from the Whitebeard fight. But, everyone assumes he's dead, so he's able to move around in the shadows, manipulating events. Blackbeard is down, but not out.
So, what does Blackbeard do in that situation?
Well, who should he turn to...but Buggy?
Buggy is still rebuilding his crew and image after escaping Impel Down. So, Buggy still needs as much manpower as he can get his hands on. Plus, helping out Blackbeard would REALLY annoy Shanks, so Buggy gets talked into it.
Cross Guild is going to be forming a little early in this timeline! Publicly, it's still Buggy's Delivery. But, Blackbeard controls more and more of the enterprise behind the scenes, biding his time. Buggy still becomes a new Warlord. But, he's helped by his new pal, Blackbeard.
Who you just know is waiting for the right time to stab him in the back. But, that's pretty hard to do, when someone can slice himself apart at will.
After our shorter timeskip:
Since the Whitebeard Pirates are never disbanded, this means Fish-Man Island is now Ace's territory. Which means, Hody Jones is now Ace's problem. Without the instability that Whitebeard's original death caused, Hody Jones' uprising doesn't gain as much traction as it originally did. And Ace is able to deal with his first big responsibility as the new captain of the Whitebeard Pirates well enough.
With Blackbeard laying low, Bonney is never captured by him, and the Bonney Pirates remain together. Still trying to find Kuma. She's able to reach Egghead much earlier in this timeline. And, after trying to fight the various Vegapunks, she gets captured, only for Vegapunk to call Kuma back to Egghead from Mariejoise so they can reunite together. Since Vegapunk probably has a recall command he can use.
Also, because of Blackbeard laying low, the Revolutionary Army never lose their base on Baltigo.
Also, Blackbeard never conquers Hachinosu. Instead, Ace ends up taking that island away from Ochoku. Pirate Island is now a pirate paradise for mostly good pirates!
And Perona never leaves Mihawk, because Moria is never captured by Blackbeard.
With her father Akainu dead, Hibari goes goth in this timeline. She still has a crush on Koby. And she ends up with her father's Magma Fruit. You now have a fiery girl, with a crush on a very zen-like guy, which I just think is a cute match.
Sabo still gets his memories back from the news that Ace is the new Captain of the Whitebeard Pirates.
With Akainu dead, that means the fight between him and Aokiji never happened. So, Punk Hazard remains a poisonous wasteland after Caesar set off his bomb, with Caesar still doing his experiments in secret. This is the Strawhat's first big challenge after finishing their one year training. And they barely manage to squeak by. Law still offers his alliance plan to Luffy to take down Doflamingo.
The Tournament on Dressrosa is now for Whitebeard's Quake Fruit. Luffy, Ace, and Sabo all reunite to try and win the Fruit, mostly fighting each other for fun. Blackbeard makes a attempt to steal the Fruit for himself, but he's thwarted, but still remains hidden so no one knows he's still alive. Sabo ends up getting the Quake Fruit, replacing his Flame Fruit in the Original Timeline. The "Three Brothers" along with Koby with Fujitora all team up to take Doflamingo down. And Blackbeard takes advantage of the situation to steal Doflamingo's String Fruit, instead. Doflamingo is dead in this timeline.
Instead of fighting Jack on Zou, they end up fighting Garp instead. Garp throws the fight, and the Strawhats lick their wounds.
The Strawhats spend a little more time on Zou to train up. But, Whole Cake Island goes mostly the same.
Because they still have their original base, Sabo's more powerful Fruit, and because they don't need to rescue Kuma, when the Revolutionaries attack Mariejois, they're in a better position. They end up saving Mjosgard, and the first Celestial Dragon ever joins the Revolution!
Once the Strawhats reach Wano, Luffy actually gets some much-needed training from undercover Garp, of all people. Much to Luffy's chagrin. But, Ace is also here, visiting Yamato in secret. Luffy, Ace, Law, Kid, Yamato, and Garp all team up to defeat Kaido and Big Mom. Luffy doesn't awaken Gear 5, yet.
In the aftermatch of Wano, Yamato joins Ace's crew. No need for her foodie tour in this timeline. With Garp's mission over, he's able to go back to the Marines. But, he drags Kid with him. He's kind of "bonded" with Kid, and decides he needs some discipline. Forcing Kid to join the Marines with Garp training him. Kid does NOT die, in this timeline, basically taking Koby's place as Garp's understudy...but he'll WISH he did! And since Blackbeard is in a completely different place in this timeline, Law's crew remains together until they reach their destination...Lodestar!
With Kaido and Big Mom now overthrown, the new Yonko are announced! It's Shanks, Ace, Buggy...and Law! Luffy still hasn't proven himself enough to get the Yonko rank, yet.
Mihawk and Crocodile still join Cross Guild, after the Warlord system is dissolved at Levelry. But, with Blackbeard now more or less the one in charge. And frankly...he's better with money than Buggy is. With Buggy as the face for the organization, being kept in the dark. Blackbeard is now able to literally puppet Buggy with his new String Fruit, making Buggy's life miserable. Blackbeard is now the one steering Buggy's journey. It's still oddly working out for everyone, though.
Gecko Moria ends up joining Cross Guild on Blackbeard's offer this time around. Oddly charmed by Buggy. Perona and Moria get to reunite AND she gets to be with Mihawk! Also, Perona helps talk them both into joining Cross Guild, because she thinks it sounds like fun. That feels like something she'd do.
After being reunited with Kuma, Bonney joined the Marines so that she can be Vegapunk's new bodyguard and stay close to Kuma. Bonney is able to uncover York's plan to betray everyone early, and helps the other Vegapunks escape the island. That's when they all run into the Strawhats. Egghead plays out slightly differently. With York chasing after everyone, instead. But, they all end up at basically the same place, with everyone on Elbaph. Since Vegapunk left Egghead, Saturn is not needed to bring him in. So, Saturn never dies. And Garling doesn't become a Gorosei...yet.
But, with Garp back, Aokiji makes a decision. The Marines are defecting from the World Government. It's something I'm not sure someone as loyal as Akainu would ever do, but it feels right for Aokiji. Aokiji and Garp team up to help the Marines overthrow the Celestial Dragons.
Elbaph mostly plays out the same, but this time Luffy and Zoro clearly fail Scopper's test in the Throneroom. So, he decides to help them train a little more while they're here. Which they have more time to do, because the World Government...collapses early!
With inside information from Mjosgard, and the expanded fighting forces the Marines bring in, and Sabo just being a lot stronger in this timeline with the Quake Fruit, the Celestial Dragons fall earlier than expected. Imu is on the run, with the Gorosei and God's Knights protecting them. They never show up on Elbaph.
Meanwhile, Shanks has a head start on everybody. And due to his declaration of becoming the King of the Pirates earlier in this timeline, Shanks has already claimed Laugh Tale as his territory. He knows where the island is, and he's more driven to claim it this time around. So, everyone thinks the end of the story is everyone going up against Shanks in order to claim Laugh Tale for themselves.
But, Blackbeard is still out there. And with the combined powers of the Darkness and String Fruits, he can puppet anyone he wants while hiding from the shadows. Including Ace and Sabo! So, Blackbeard could still gain access to the Flame and Quake Fruits, as was his plan in the main timeline. And maybe even Imu, now that Imu's outside the safe walls of Mariejois! Blackbeard's gearing up for his revenge on everyone!
...And that's where I'll leave things. Everyone against Shanks for the title of King of the Pirates. Luffy and Blackbeard both in weaker spots than they currently are in the main timeline, but both growing in power and allies. Some things are different, but a lot's still the same. It's not like Sengoku or Akainu have made huge contributions to the plot. The biggest change is Ace still being active, but he's kind of off doing his own thing. There'd be more team ups with Luffy, which means the Strawhats don't need to be as strong.
This...actually kind of lead me to a timeline where more things work out than normal. It probably all goes crazy by the end, because things didn't go EXACTLY as they had to in order to work out. But, having Ace around kind of lets Ace deal with almost half the problems that Luffy would have dealt with on his own. Weird.
