Here's something I thought about.
The WG showed concern over Whitebeard and Shanks forming an alliance. As one of the gorosei pointed out, Shanks isn't the type of person to instigate problems/trouble, so that fact put them at ease. It's also evident that the aforementioned 2 emperors enforced order in the New World.
However, with the death of Whitebeard, someone has most definitely come along to occupy his vacant seat amongst the Yonkou. I've said for a long while now that that person is going to be Blackbeard. Given how Blackbeard has shown himself to be traitorous and conniving, not to mention how he was the source for the trigger of the greatest incident to occur thus far, the WG will be keeping a close eye on his movements.
Now, what if at some point in the story, Blackbeard and Kaido share a common goal and decide to team up? I suspect that this alliance would potentially be far more threatening than that of Whitebeard+Shanks, because unlike the latter two, these pirates have malicious intents. I'm sure the WG would have the foresight to know that such an alliance can be disastrous for the world. Chaos and havoc would reign supreme.
So guys, any thoughts on this possibility
?