
Teach was a child forced to eat his own family during God Valley to survive. Whitebeard spent his life giving food back to his family. Blackbeard at his family to survive. Same way he ate Whitebeard's heart to get the Gura Gura Fruit. Teach was saved by Rocks D. Xebec from God Valley, but, has never been able to sleep, because he is cursed for what he did to his family.
Because of this, Blackbeard worships Rocks as his hero
Buggy IS Rocks son, and he went missing after his father lost in God Valley. Why? Because he was stowed away on Roger's ship. Rocks' wife knew that her son would be hunted by the world, so faked his/her death, and then gave Buggy to Roger to raise. This mirrors Roger giving Garp his son Ace to raise.
Shanks knows this, but has never told Buggy, because he knows Buggy would brag about it and get himself killed like an idiot.
BUT, if Teach finds out...
Blackbeard Pirates + Cross Guild vs Mugiwara and Akagami Pirates

If Teach is the kind of Rocks stan I believe he is, and Buggy is Rocks' son... THERE WE GO!
Teach: "ZEHAHAHAHA, I ALWAYS THOUGHT YOU WERE WEAK, BUGGY! But, I was wrong! You're not a killer... you're a king! A survivor! The kind of ambition those government scum just can't crush! The harder they hit, the higher you go, Buggy-sama! You've got the two greatest strengths these waters can offer; Luck, and Charisma! Zehahahaha, your men really love you! They'd even die for ya! I love it! After the Marines promised killing Pops would bring peace... they've only allowed more war in the Grandline! They killed the one man, Whitebeard, who was willing to do their own jobs for them, for free! Zehahahaha! Akainu's a fool! That's why the people love Cross Guild! You're a hero, Buggy-sama, to all the people who've been screwed by those Navy bastards!
"Join me, Buggy D. Xebec, son of Rocks! We will be his saber, even in death! Together, we will inherit Rock's legacy, find the One Piece... and claim the entire world as our kingdom!!! ZEHAHAHAHAHAHAHAHA!!!"


Oda's been planning this for a long time, IMO.
-This also allows Zoro to fight Mihawk at Laugh Tale.
-Mihawk and Croc get Blackbeard's protection for Cross Guild
-Teach gets paid money to kill Marines
-Teach gets another Yonkou on board and now controls the entirety of the New World Underworld
-Buggy gets a legit Yonkou crew to sail to Laugh Tale with, with a dude Shanks is wary of, Teach
-Buggy gets his own ship/fleet under Teach
-Mihawk gets to hide behind another Yonkou to duck Shanks
-Croc gets an alliance that's easy to get out of, since he doesn't even want One Piece, he's just using Teach for power
-Since Mihawk and Croc want to build their Utopia, not find Laugh Tale, they have no problem letting Teach reach Laugh Tale
-Teach knows Mihawk is NOT someone to fuck with, and will offer incentives to make it even more worth Mihawk's while
Buggy is the son of Xebec, because he has the same power Luffy and Rocks had: the ability to gather people around him.
That's what makes Buggy dangerous, his Rizz.
Wano Act 3 was a weak alliance doomed to fail
Wano Act 4?
It's Buggy and Blackbeard time, baby
SUPER FLASHY ZEHAHAHAHAHAHAHAHA