As a start for this game , Races and boxing would be very easy to add.
For racing im thinking besides the curent races in the game , a custom race making feature would be useful and would keep the game fresh , i'm saying if there'd be a flying camera , a menu to choose what vehicle would the race have (bikes , karts etc.) , a starting place marker , a finish marker and a placer for the guiding semi-circles (like in the original races) would be the only things necessary (props , ramps , obstacles etc. would maybe be good as a future update).
Boxing would really only need the adaption of having 2 boxers instead of 1.