The Unity ports of DooM have a coop feature, I'd love to see a category for co-op runs if possible.

The reason it was never added was because of the online feature of it being complete trash. Otherwise we would probably have another board separately for it.

If ever Unity's online improves, I'm up for it.

if someone were to do a split screen local coop run and submit it to category extensions do you think it eould be added?

No. As I said above, the online feature is what kept us from bothering with a co-op board. There aren't enough runners for co-op to justify making a board. Plus, the cat ext. board for this game is reserved for the add-ons within the game (excluding Plutonia/TNT) and for the community-made wad we have.