ZDoom

This is a small mod which shows small radar on your HUD. It can detect enemies, projectiles, items and show them with different colors
It has 2 modes, which can be toggled by button (configure it in menu)
Requies GZDoom 1.5.6 or ZDoom svn r2532+
Screenshot:

Neat! I can imagine this would be useful for a Marathon TC.

Methinks Tragos and co. would find it handy, too.

Could there perhaps be a version that only shows enemies if they're moving?

Yes, i uploaded new version with this function(toggle it with button) and ACS source

Literally not compatible with any other mods

Skittle wrote:Literally not compatible with any other mods

What's the problem specifically?

It replaces all monsters and even the player.
Unsurprising, considering this mod is from 2011. There wasn't really anything else you could do back then, universal mods weren't really a thing (with extremely few exceptions) before ZScript