Also Oda turns into G.R.R.M. (who can write 1 chapter a week), plot power and Nika are defeated.
The biggest plot point would be that Kaido managed to defeat a prophecy bending fate, heavenly will, Nika is not reborn, the child of the prophecy is dead. Now I would have 2 scenarios:
1) Luffy keeps his plot power even if dead and every "good guy" friend of Luffy is pissed at Kaido and BM. A big alliance of justice is made to take revenge on the evil Kaido. I would put in basically everyone in the manga who is not evil or neutral lol. This alliance would have Dragon and the revos, Shanks and his crew, Garp, Boa, old Ray, all the kingdoms allied to Luffy (Alabasta, Dressrosa, Fishman island, Water7 etc.), then there are big maybes that could actually don't care at all or be enraged like Mihawk. At this point the WG could even ally with Kaido and BM but that is stretching it too much. Big battle but in the end the alliance is too big and too strong, Kaido and BM are defeated by the world is still doomed to fall under Im or BB influence. Also about BB I don't think he will organize his plan so badly so that BM has the time to return to WCI to stop him, also she would be exhausted after the fight so I don't see her outsmart BB..
2) The world works according to logic: Kaido and BM are a huge threat, a lot of guys are very pissed at Kaido and BM (see the above) but making an alliance is difficult plus now there is a big incident either way.. Kaido and BM are weak and GB is in the area, he tries to attack them, considering everything I would say he manages to take out some injured YCs before he even manages to escape. The real problem is that there is also Shanks in the area.. he could very well wreck both the injured Kaido and BM, he also has a good reason to do it (the killed the golden boy he gave the straw hat to), actually he would most likely do it by that point without caring so much and go look personally for BB after that (this would actually be pretty cool, Shanks takes the lead being a more ruthless MC than Luffy just like he was with Kidd, and he wasn't even totally pissed). Another fun thing could actually be Shanks and GB allying in order to kill Kaido and BM, Shanks is assured they will be killed and not captured while GB takes the credit.
Either way this Kaido BM alliance would be short lived and don't change their fate too much. From that point depending if Shanks is bloodlusted and what relation he wants to have with the WG he could even jump GB after Kaido and BM are done. Anyway the danger for him will keep being BB. Just thay this time he must act alone (if he actually can do it) and there isn't much time since BB gets stronger and stronger. At this point maybe they could clash and either BB or Shanks is killed. But the other is weakened just like the WB remnants after Marineford (the winner will be heavily damaged either way). Now they can be jumped by the WG, the revos and even Buggy depending on his guts and if Mihawk and Croco agree. But this stuff needs a closure. Both BB and Shanks die either way. Now Buggy will try to find the OP alone and eventually succeed or fail but the point is that he isn't the child of the prophecy so nothing will happen and even Im wouldn't care about it. Oh, Vegapunk is killed by the CP9 according to the plan. The revis and the WG are left, the WG is far stronger, the revis are eventually obliterated. Im reign keeps going on forever (or at least until a new child if the prophecy is born).
If we want to go on with the Kaido BM alliance let's say Shanks is not in the area and same goes for GB. Everybody gets a feast after the arc and recovers (typical SH ending of an arc). Now the WG is relieved one of his 2 big targets is dead (Luffy) but the Kaido BM alliance is still a problem, for sure comparable to the former Rocks crew. BB adopts a low profile, maybe even disappear like the Revos did with Baltigo, until somebody has dealt with Kaido and BM. Shanks is destroyed by luffy death but his target is still BB (just that like before he now most likely has to deal with him personally). At this point what could Kaido and BM do? Let's say they manage to cooperate peacefully and don't try to kill eachother at every occasion. Since Kaido was interested in df I guess he tries to steal the secret of stealing df powers from BB crew (ala kills a df user and have a fresh fruit close by ready to absorb the power) and most likely manages to get it by bribing or beating an high level BB commander. BM could try to accomplis her dream to ally with the giants and even if they are strong they are still weaker than the Kaido and BM alliance, in the end she enslaves a lot of them, the others are killed (and probably the island and the tree burned).
Now they would just crash in the WG who is still too big for them.. the 5 gorosei, admirals, holy knights etc. attack all of them together. They could even made an agreement with Shanks and BB to have them do nothing while the WG deals with Kaido and Bm (BB is not trustworthy and could very well backstab but it is also true that Shanks could use this chance to attack BB)(and there are still the revos but they too are pissed at Kaido and BM and want them dead so who knows). Or just have a single group (admirals, holy knights etc.) stay behind to defend the fort while the others attack the Kaido Bm alliance.
