Spoiler: (Open)I use to play a lot coop Doom with a friend. Something that always bugged me was how you could not tell at a glance where other players were, so I decided to see if I could put in something like the arrows from the Serious Sam games.
This has been sitting unworked on on my computer for months and I simply don't have the energy to work on it further. I think there were some rough edges born from me not fully understanding the netcode ACS stuff, but it seems to mostly work. I am releasing it in it's present state with the Unlicense license. Do what you want with it. Hopefully someone finds it useful.