Dissidia Final Fantasy is out in North America, and I've been playing waaaaaay too much of it, but here are some of my musings on what would be interesting additional characters.
I'll try and keep the list limited to characters that would be fun to play, but some, er, punching bags may have also made it into the list.
This game was obviously created for Final Fantasy VII fans. If you didn't obsessively follow every plot point in Final Fantasy VII the story will be confusing. If you don't remember what Jenova is or how Cloud relates to the Sephiroth clones, you might want to find a story guide before playing.
If you never played Final Fantasy VII and have no plans to, then forget it. The story will make no sense, the cutscenes that exist solely for Final Fantasy VII fans will mean nothing, and the gameplay just isn't fun enough to make up for this.
2 (Poor)
4 (Good)