So to begin with, Thriller Bark is where Oda actually starts consolidating some of the concepts he built for years since the beginning of the series
Such as one of the most important and proeminent ones: The Monster Trio
So that said, i think Oda might already have given a pretty obvious clue of which kind of enemy they are going to face EoS
-Zoro is going to be the WSS, so his Top Tier opponent needs to be Mihawk
-Luffy is obviously going to be the Pirate King, so he will fight whoever has a chance of stopping him from achieving that.
This one might be the most ambiguous one because there could arguably exist more than one possible final opponent for Luffy here, the most obvious one for most people would be Blackbeard, but Akainu or maaaybe Imu are also possible options, since they also might end up being one Luffy's biggest pain in the ass preventing him from achieving his goals (especially Akainu tho, in my opinion). Anyway, he is the main character so you can expect a lot of things
-As for Sanji... He hinted out that he will give lot of trouble to the Navy. In that case, whats best than managing to actually beat a Navy Admiral?
Have you guys noticed that Oda is delaying Sanji meeting a Navy Admiral in the post timeskip? Well, there could be a very good reason for that and Oda is waiting for it