I would take Reflex, and somehow try to make it slightly easier (or funner) to just mess around on for a beginner, while at the same time being more difficult to master for the expert gamer. this can be done with improvement to the physics over time. The trick is finding the right formula to simulate the suspension of the bike and gravity on the track. I'd focus on the whip effect of turning the bars, and the back end kind of floating/ pivoting around the head tube and steering stops in the air over jumps. (Again, they were on right track with Reflex, but as in the next generation of this.) Next would be improving the feeling of drive from the rear tire. Finding a way to have the back end step out under throttle, and to be able to counter steer to correct. But being able to have the back end step out be a good thing, i.e. steering with the rear wheel. Maybe a manual transmission, but that's a reach, not enough buttons. Lastly (again building on Reflex) find ways to put more small details into the riders mannerisms while performing maneuvers (such as a foot off the peg when scrubbing, right leg moving when you use the rear brake, etc). The idea is the more small details like that, the funner the game is to play. For example, a nice touch would be if you were holding the clutch button and the throttle open, and then released the clutch in the air, the rear wheel would suddenly spin. Or on a hard brake check in the air, the suspension would slightly compress with the gyro effect like on a real bike. I know the way the tires interact with the ground and the movements that the rider makes have to basically work the same, so this is all tough to do, no question.
Next thing most important would be the artwork, or the background of the environments. Of course, the perfect game would license the outdoor tracks. Re-create the tracks, and they will be fun. But really re-create the atmosphere of the track, to scale. That's a main thing when thinking about how the graphics should look, scale. I think in the ideal game, the rider/bike takes up more of the screen, shows the rear suspension working, and the rear tire locking, sliding and clawing. In general, the game appears to be "moving slower" compared to older games, yet at the same time delivers the sensation of speed for the player. Like if you go off a jump leaning too far one direction, you are floating that way, unless you turn the bars and lean the right way, and apply the right combination of throttle to land how you want to land, just like on a real bike. The camera perspective should be "looking up and out" into the environment as much as possible instead of "down onto", for a large sense of scale. Maybe this game is better suited for playstation 4, lol. But that's my megalomaniac idea for an MX game.
As far as profiting on it, if the tracks are really great, hold back red bud and glamis or something, and charge $15 for the DLC packs. Continue making new tracks after the game is released at $10 bucks a pop.