First off, any console ports will require splitscreen support.
 * Player code needs to be encapsulated inside of a player class.
 * THE GLOBAL STATE MUST BE NUKED!

After that, figure out rendering a second player's stuff in their own viewport or surface. Or something.

Controls for multiple players on one physical machine (controller support)
Stretch goal: multiplayer + local co-op if possible = yay

Sound, music, etc for multiplayer players

GUI scaling is critical.
Cram everything so you can fit 4 players onscreen.