If you're choosing to do this because you've got a really clever story in mind and you think the result will satisfy the recip, that's fine, but if you're just looking for a loophole that will let you write something other than what you offered, that's shitty.

If 95% of my gift was about some bullshit OCs, I'm gonna be annoyed, even if you do the Scooby-Doo reveal at the end.