Considering how much money and work would have to be put in making the game, in order to really do something ground breaking and make the game exceptionally more enjoyable to play than skate 3, makes it hard for any game company to get interested on the idea. The truth is that the skaters and people interested in skateboarding lifestyle aren't really a big enough group of people that would buy the game enough to cover like 50 million dollar development costs.
IMO It would have to progress a lot into the GTA direction, and should be easier to pick up by gamers who aren't obsessed in skating, while still keeping the real skater interested in it. Every kid liked the THPS games and it was easy to play and got them interested in skating, now that kind of effect would be a lot harder to accomplish.
It will take a few years before the pressure for a new one builds big enough and somebody decides to try to make it happen, and I hope it's gonna be good.