Spoiler 
What hasn't been done in a 2D sidescroller, and a team based game? Well, how about BEFRIENDING the enemy?
You choose your ally. You fight with your ally. Anywhere. Anytime. Anyhow.
But with every good perk comes also a bad one. Die too much to one person and he'll become your nemesis!
More 
Use the included AllySystem5.lua as your mp_luaserver or add the line " dofile("sys/lua/AllySystem5.lua") " to your server.lua . Also copy the graphics to the respective graphics folder, and use the included serveraction.lst file in order for people to see them.
The script uses hudtxt ID's 1-10. If you want to change them to avoid collisions, go to the top of the script and edit the table "as.idtable".