Actually, I'm fully expecting Splinter and April to be playable in the final product, even if as unlockable. As I mentioned, this game is being made by the developers of Streets of Rage 4, and that game has
a lot of unlockable characters and retro elements.

Not only that, the Genesis version of TMNT Tournament Fighters had April (the SNES version had Karai) and she was modeled off Blaze from Streets of Rage, so it feels extra appropriate to have April here.
Nick and Paramount have given Splinter and April more of a role in the action pretty consistently in their era of TMNT (even renaming the group "The Mad Dogs" in Rise of the TMNT so they can have April as the 5th member of the group) so even though it's not particularly reflective of the original cartoon, it WOULD be surprisingly to go that route here.