I wouldn't count on seeing Deadpool anytime soon. The X-Men and Deadpool are owned by Fox in a way, so the rights may or amy not be up in the air, though that may be only for cinematics. I can't say if that would apply to video games or not... It didn't seem to matter in the Lego Marvel game, so...
Dead pools presence would explain why the games plot is so messed up and therefor would not work because without I stupidly confusing plot KH just isn't KH anymore