I'm a bit surprised this hasn't been done before... or atleast, done enough to reach the point of "Beating a dead horse."
To be honest, I had been wanting to draw Sora and Kairi in those adorable wedding themed Mickey ears you'd always see at least one couple wearing... (If anyone wants to see it, let me know and I'll link ya)
If you ever decide to make more Heartless, would you mind making Kurt Zisa?
For some reason, that thing's gained a lot of popularity in recent times.
It stands out because, with Sephiroth being 1st, it is the 2nd most difficult battle due to it having multiple fast attacks and a health bar equal to the Final Boss of KH1.
It originally started as a contest, specifically a Name-In-Game contest, where people would design a Heartless for the game before its North American release.
Kurt Zisa won, and it's hit the popularity counters ever since.
Check out it's wiki page -> www.khwiki.net/Kurt_Zisa
Note: Kurt Zisa is named after Kurt Zisa of Medford, New York who won the contest.