Earlier today, I was setting up for my race, and I started thinking about a potential iGP career mode. I don't know what exactly would be necessary to do something like this, but it certainly fits within the schema of the game.
I think a career mode would be like a series of beginner to advanced tutorials. Starting off with easy races at lower levels, then earning some amount of XP for each career race and leveling up to face stronger threats. This would be an optional mode of course, though those who play it would ideally enjoy more success than those who don't.
I also think that iGP should capitalize on its online race mode, as that would be the way to more accurately gauge the rankings and reputations of different players. In a league, you're only racing most of the same people over the course of a season, but public lobbies allow you to go toe-to-toe with players around the world. There should be cash and maybe token rewards for this.
As for actual league racing, I think it's good where it is. I like having a healthy competition with my friends. I just think these are two things that can be tied into the game to improve the experience.