One thing I've always wanted in Street Fighter is a way to model characters who study multiple styles. This is an idea for how to do that without making major changes to the system. If someone has already done this, I'd be happy to hear about it. And if there are obvious reasons this wouldn't work (or simply issues that would need to be sorted out), I'd love to hear those too -- I'm dashing this post off pretty fast, but I'll try to get back with replies if there are any comments.
The basic idea is to simply to a) add Style ratings to the character sheet, so characters could be level 4 Karate, level 3 Kung Fu (for example), and then b) treat the power point costs listed for the maneuvers as prerequisites. The special maneuver "Dragon Punch" has a power point cost of 4 for Shotokan Karate and 5 for Kung Fu and Thai Kickboxing. This would translate to a simple requirement that a character must have achieved the 4th level of mastery in the Shotokan Karate style to get access to dragon punch, whereas a kung fu stylist would need to be 5th level in their style before getting access to the same maneuver.
Using this system, characters would need a starting pool of points to place in different styles, with a standard maximum rating of 3 (before freebie points). How many? I'm tempted to simply say 3.
The "any" category would simply default to your highest level in any style: if you have a 3 Karate, you'd also have access to any maneuver from the "any" list with a cost of up to 3 power points. This is a fairly simple system for allowing characters to be masters of multiple styles, and it works best if you're taking a "style proliferation" approach to the martial arts in the game. With this system, you could create Ryu as something like Karate 5, Tae Kwon Do 3, Judo 3, and so on.
The system isn't perfect: I haven't checked it for breakpoints (why go to level 5 in style X, if there's no maneuver with a cost of 5 in the style?). If you're using standard experience point costs and rules, it also makes the power point costs doubly important -- they limit access to maneuvers which also will cost more when access is gained. If you're not using the regular SF:tSG xp system, this problem can be avoided.
The system doesn't only restrict, however: it also opens up options for characterization and for character building (in terms of combat). Previously, there was no way to build a character with certain combinations of special maneuvers: you had access to the maneuvers in your style, those in the any list, and that's it. For story based reasons, it was sometimes desirable for a character to be able to learn a particular maneuver, but the only way to get this access was simply to make an exception to the rules on a case by case basis.
That's the basic idea, essentially back-formed from ideas for multiclassing in the d20 street fighter hack I'll also be talking about sometime. As always, I'm open to suggestions.