@ Nova i would assume it comes from the fact that WB had 43? allied crews. Not to mention the division commanders. Considering that we are at 7 allied crews it would make sense to expand. I don't really ever expect it to get that way though I don't think there will ever be division commanders because Luffy isn't gonna have a bunch of random people join.
As for my picks Germa, the WB pirates, Minks, and Samurai are all options after Wano.