There won’t be a villain cover, that’s why the Beast Pirates got that colour spread a few chapters ago.
I think if that was going to happen then either Chopper or Usopp would have been saved for Volume 101.
It’s all about the balance. Since it’s three conjoined covers, I’m expecting each of the three to have basically the same layout. Only three characters in the middle one would make that one look empty, while the other two in return look too packed- and the numbers and height ratio would be wonky as well. It would either be 5 character, 3 characters, 4 characters (no Yamato),
or 5 characters, 3 characters, 5 characters (with Yamato, but then there’s three really tall characters in the final one, Brook, Yamato, Franky, plus Nami, which is just wonky and unbalanced)
To me, each cover being one person in the back, four people at the front makes the most sense.