I would hope for a few things to make this an instant buy for me.
1) Have the entire multiplayer experience unlocked from the beginning.
2) More levels. Hey, I know that might be too much to ask, but a few new levels (One for each path would be nice.)
3) Map selection of path. I know the Nintendo 64 had an awesome branching feature but it was hard to get to the other levels, let us pick our path.
4) Multiplayer single player mode. Ok, here is the big one. Make each level have a few multiple paths to take and allow for 4 player single player action. Have those paths cross and sometimes and require everyone to help each other would be a great experience.
5) More free movement levels.