Shredder and Splinter seem automatic. I would hope that April wouldn't be made, but she might. If nothing else, villains would be needed to support Shredder. How can you have 4 turtles + April + Casey + Splinter going against Shredder all by himself?
There's also a question of whether wave 2 would include 4 figures. Just because the first wave did doesn't mean the second wave will. The first wave had shared parts. They may go the Bandai route with smaller waves.