Copyright infringements would be a problem. Just cause we have one game with them doesn't make it any less of a problem. Unless you redraw all the guys yourself flash-style. Then it would work.
I actually would be more concerned with making the game than worrying about copyrights First of all do you realize how many copyrighted game remakes there have been, all's you have to do is put something like "Flash" before the title name and you're good. Portal the Flash version is the epitome of this.
Aside from that I will say that 2D fighter games are pretty stinking hard to make actually, that is why you don't see them in flash form very often.
Well, they don't mention GlaDOS in Portal: The Flash Version. Portal is just an everyday word, so anyone could use it. But the creators couldn't name GlaDOS because that is a Name made by Valve and it's their right.
More to the point, I'm sure someone could make a MK game, though I think it'd need to be called Mortal Combat, as opposed to Kombat, unless the creator doesn't plan on making any money off of it.
Yah, but I think "GlaDOS" infringement is what would hold up in court.
Besides, the original Valve team didn't come up with the concept. Some people in a college did, and when valve saw it they hired the people and made the game.