Don't know if this thread is dead, but a few things:
Step-up landings, like the ones in the X games 2012 and 2011 big air jumps.
Character customization, just the option of changing the colors would be dope. Same goes for the features, like if you could change the color of the panels or the paint on the jumps.
Add tube rails to the level editor.
A few new grabs would be nice, there's plenty of space on my keyboard ;)
Some requests: superman (not a grab, but still...), Liu Kang, everybody seems to want stalefish, double mute and dub japan(could be done by holding Z+A and C+D), octograb, Salomon grab (like a high safety, but your legs "scissor" like in a japan.
It would be dope if you could get double grabs by combining several old grabs in the fashion I mentioned before. Like if you added a Salomon grab you could combine it with a tail (technically it should be a toxic, but waddever) and get an Octograb.
Wipeouts, or just a simple forced restart when you go too big and land flat, or if you don't land facing the direction you're going. I can see why this could be a problem, though, for example if you are shredding a hip it might be hard to program the game to admit 450s on those landings but force a restart on a regular jump...
Maybe the option of resizing features in the editor, for example shortening boxes to make them fit into the feaures you are planning. This is not a pressing matter.
This is all I can think of right now. Great game, playing something like this is truly a childhood dream come true!