One of the changes I'd like to see is I feel like the system should weight playing at a higher player count higher. You can't go off time played as how would it work for turn-based then, and I'm not sure how say how the system for win % works would work either as that might weight it too far, as not in every game is adding a player linear in the amount of time it adds.
There is a clear issue though, that if you play 2 player you have a significant advantage at qualifying for gurus under the current system. It isn't a problem for Heat currently, but I can see how it could become one, and therefore how it might be one at other games, where you can play a 2 player game in a 1/3 or a 1/4 of the time of the most popular ways to play the game, yet if 2 player ever become the Arena format I think it would damage interest in the game, because gameplay is just nowhere near as good at 2 player. Indeed one of the reasons it went from 3 to 4 player was because players didn't like the gameplay at 3 player, and there is debate to change it to 5 player, with time being the main concern for not doing it rather than gameplay, as pretty much everyone agrees gameplay would be better.