@EvoWarrior5:
Yea but as I said, he'll want to take them all out eventually. Big Mom will be severely weakened by the war as well.
But I guess that since he is also preventing Kaidou from re-building his army he wants to specifically focus on him first. Even if both of them are weakened, no matter how much, he probably wouldn't be able to take out both.
It's the whole "cut off Kaidou's resources" thing that makes me think something bigger is planned next than Law and Luffy sneaking into Kaidou's bedroom and punching him.
The way to attack a guy with huge ass armies of actually strong stuff would almost definitely not be "ok now we go on a frontal assualt". It would be to get Kaidou's whole empire in danger.
A war with another Yonkou seems the only plausible thing to do that.
And Shanks and Blackbeard are too damn important to be that other warlord.
Also if we have to deal with all the Yonkou before the end in some respect, especially the two less important seeming ones, a two for one larger storyline makes good sense for Oda to choose.
Note he also introduced Kid and his alliance after some Yonkou.
Now lets look at the possible Yonkou.
1. Shanks: No. There's just no way. They are either defeated completely or…well that's the only thing that will happened. No way is Shanks being taken out by Kid. But likewise why would Oda piss away thee Supernova crews to just reinforce what we already feel, that Shanks is badass motherfucker.
2. Blackbeard: No. Roughly the same reason as above, though somewhat more plausible in regards to Kid and co being crushed in the attempt. Mainly because it could give us a good snapshot of just how fearsome Blackbeard is. BUT why would Oda piss away three crews like that, there are far better ways to show how scary the BB's have gotten.
3. Kaidou: Possible, but considering Oda has already basically told us Law has a plan to take out Kaidou? And not just some vague aim at Kaidou? That it wouldn't make a lot of story sense. The plot right now is a tough plan that ideally with take out Kaidou. Having the Kid alliance join on on that would only make things easier in a pedestrian way "Oh hey guys, we were on our way to do the same thing", and the idea should be to make this as tough as possible. Carrying out Law's plan.
4. Big Mom: Most likely. If this whole storyline expands into a war between Yonkou, it would be setting up two storylines to run at the same time. After (or during) Law's Kaidou plan, and it might even be that Big Mom is taken out not by Luffy's involvement at all. Which would be a tad more realistic. And also set up Kid as a bigger rival.