I have very few ideas, seeing as almost all of my ideas have been used. The one hit kill idea I had came to reality with Guilty Gear X. Many games have utilized the team idea (I'm not sure which was first). The only good idea I have left is the battle royal game of the century. Basically, the core idea is take some characters from a lot of the current fighter games (Guilty Gear, Tekken, King of Fighters) and put them in one battle royal. There should also be some teams in there. This would make the gameplay much better, because you could pick the people you're good with in other games and put them on a team. Thus, with the right people, it would make it very interesting to see the results. That's all I have. Anyone come up with anything better?
I have a few ideas. One is the adventure mode, which Namco came pretty close to in Soul Calibur 2 with Weapon Master mode, or whatever it's called. This, however, allowed you to switch between characters in the middle of the adventure and the stories didn't offer specific insights on the fighters you were using, the ones you'd grown to love. As a general rule, most fighting games are forced to keep an individual's plotline seperated from the gameplay itself, leaving it only for the ending and occasionally a short beginning. The ideal adventure mode would offer more in-depth stories on each fighter and, while I hate to make the comparison, would function like a Grand Theft Auto game: you're given a city with multiple courses of action for you to choose from. The game could be different depending on your path, and would inevitably lead to a worthy ending.
This may be a little far-fetched. The problem with such an idea is that fighting games today have at least ten to fifteen characters to choose from, often many more. There is no way for the programmers to make unique storylines and levels for each one. If there was one main storyline that all characters took, it would not allow for much personality between them, as the plot would have to be general enough so that it could apply to any of the diverse characters included in the game. The only solution would be a carefully-orchestrated compromise between the two... many levels would be such that any character could play, but a few special levels and many personalized cinemas would be exclusive to some players. I think it could be done.
My second idea is a Pokemon fighting game. Nintendo's abundant use of Pokemon in Super Smash Brothers Melee shows that they have great potential, what with 150 (or whatever number they're up to now) characters to choose from, many with spectacular abilities. The entire idea behind Pokemon is to have them do battle, so why not? I can see a tag-team system, as is very popular nowadays, with the possibility of six to a team. The only complication is the thought of violence in the games... little animations of a pixelated foot going across the screen in an RPG battle is one thing, but pretty much all fighters should have a T-Rating for violence, and Pokemon has a big reputation with small children. Perhaps it shall forever remain a pipe dream to me.