Me idea:
Kil icons
You know how in Minecraft or Terraria, or even Killing Floor, they say when and how your friends died.
Well, I think something like that would be awesome, especially in the form of kill icons! Kinda like in TF2 or the Death Match games.
I think it'd be possible, since both players and NPCs have points.
I personnally can't code anything, but I can help with the kill icons.
Maybe kill icons for when you kill a monster or at least boss, but not for maps like the one with waves of Antlions, it'd be waaaaay too annoying.
Something I did as a bit of fun on Moddb that got no replies, but I repeat the OC related bits here.
One would be some sort of persistent leveling mechanic. Alot of co-op games and mods involve playing the same maps over and over, it would be nice if the player changed to make future experiences differ in some way.
That brings me to point 2, the ability to swap weapons. eg. replace a crossbow with sniper rifle. The reason for this is that, as the mod get's more and more weapons, it's going to be harder to see the new stuff, and rare that you do.
This leads me to an idea I've been kicking around for a while. A map that can be updated to show off the features of each major release. Let's say it's a walkthrough map with all the new weapons and NPCs.