Tony Hawk's needs to go back to its roots big time. Keep it simple and clean.
Problem there is, Activision can't be trusted to make that happen. It'd need a cooperative effort between themselves and a passionate, invested party. The Crash Trilogy is an example of respecting what was done before (regardless of how 'okay' those games were to begin with) with a secondary player (Sony).
Same goes for SKATE, which has just as much chance of a successful return at this stage.
An indie dev or small studio would do well to try and step into the niche left by these dead giants.