I agree 100% with the OP, I'll add a few points as well.
-60fps is madatory on 4player split-screen
-Fix item managment. Whith the exception of MKDS, after MK64 I feel every new game has done it worse than the previous. More specifically, less blue shells and don't make the player lose his items when he gets hit.
-Try to make the handling easy to learn / hard to master, more like in MKWii and MK8, unlike double dash and MK64.
-Make the battle mode fun again, and since they are it try to add some new game modes as well.
-Make avaliable for sale all the old tracks from the previous games as DLC.







