I can highly recommend War Of Rights. Multiplayer only American civil war fps.
You form a long line in a field with 20 odd men facing the enemy line. It takes around 25 seconds to reload your gun, while the enemy is firing at you.
“Gentlemen, take aim, steady, steadyyy, steadyyyyyy, FIRE!”
The whole team gets penalised if you die out of line or duck for cover.
Sure you can play doom 1 and 2 campaigns in co-op. No need for modding. You don’t even need a network card. Just buy a cable to connect both serial-ports on the PCs.
That was my first co-op experience and it still one of the best I’ve ever had.