2 player chat
T by itself - Player 1's name and chat text
T+(players number eg 2) - Player 2's name would appear with chat text
This would come in handy for people with split screen.

A built in Tileset editor

When you click a persons name on the list in multiplay it brings up a personal details menu like age and where they come from to save people from asking where are you from whats your age and stuff like that

Eva as a playable character (she is in my fangame)

A built in uploader. When you start JJ2 it has and upload menu which allows you to upload levels quickly and easily without any hassel

This sounds stupid but because there Vertical and Horizontal spilt screen how about diagonal for 2 players only to make it a bit harder to see