Well, to be honest there was a boom of attempts to create differant fighting game franchises out there for a while but most of them wound up dying, despite having some interesting ideas. In the game "Pray For Death", Death (in this case the Grim Reaper) is the final boss, the idea of that one being a bunch of beings fighting in a once in however long it ticke's Death's fancy tournament for a chance to come back to life. The cast included everything from a Bruce Lee clone to Great Cthulhu (yes, really) as playable fighters. The problem was that most of them were dialed in and the guys doing it, didn't put the love, polish, and attention to detail into them that Capcom, SNK, and Midway did.
There is room for new fighting franchises, but it would take a surprising amount of work to do right, and that's something most game companies and developers don't want to get involved in.
That said just in terms of crossovers, there is still a lot of potential to be had in the "fanboy dream" of Street Fighter Vs. Mortal Kombat, which it still surprises me has not happened yet.
Also as far as fighting games go, I've always thought it would be kind of cool to do something like "Mortal Kombat Vs. The Real World", liscencing would be difficult though I suppose. Back in the day you had relatively normal guys fighting in this tournament (and I guess you still do). It would be amusing if you dragged up the usual cast of baddies and then had a team of real fighters, the guys who might be sent to represent earth if this actually happened, as the rest. Pick a few of the more long running and dominant UFC guys, a couple of Chinese or Japanese tournament champions, maybe a Pro-Wrestler or two for Lulz ("he's for real, but has to pay the bills you see... like Johnny Cage"), maybe the current heavyweight champion of the world (but the person doing that would likely expect such a huge payday it would be doubtful). To me that would be a really amusing idea if done like a fighting game, rather than a sports game. I mean in theory if some kung-fu guy from a temple was the dominant force in Mortal Kombat, I suppose with the current reality fighting obsession you could probably sell some of the guys out there now in a similar role (purely for fun).