My predictions are all over the place but I think one ir more of these:

Morgan Sith Castoria- first three because they're fairly popular and this would be about the time they'd get one

Serenity- lower star servant that hasn't had much attention

Eresh or Spishtar. Probably Spishtar knowing Eresh's luck, also because yes Ishtar has already had one, but Spishtar and Ashtart are separate entities like how Skadi was for Scathatch.

Koyanskaya? Maybe both as one? I feel like we'd get something special for one of them at least, also to fill a villain quota like Douman.

I'll leave one of the 7 as a wild card like Lady Avalon/FeMerlin or like the summer Valks. Although I'd really love to sneak in Jane somehow too.

And for guys costumes I'll say:

Xiang Yu- not to count officially towards the actual costume change portion. I just think it'd be funny if they just gave him a Hawaiian shirt and that's it, like bunny Shiki

Muramasa- another sort of villain quota, also to pair with Castoria and Eresh/Spishtar Jason Bartholomew