What if past characters (be they protagonist or villain) began to appear, perhaps in expy form, as recurring bosses for a while? Let's say toward the end of the game, you had to fight bosses such as Seraph Sephiroth or Kuja, and they used this treatment for a number of entries in the series. Would change your mind about them becoming summons? Would you even at all approve of them being used as bosses, optional or otherwise?
EDIT:
Ansem... to clarify, if the character from KH were to become a recurring boss or a one-time summon in FF, I'm pretty sure they would just call him him Ansem and not Xehanort's Heartless.