Neverwinter Nights and Neverwinter Nights 2. I mostly played 2 in co-op, and it kinda feels like the second player is tacked on (which leads to some unintentionally funny situations), but the writing is the good kind of bad, so it just adds to the entertainment value. NWN1's main campaign is kind of dull, but there's many player-made campaigns for it.
Armed Assault 2 has a full campaign and all, but it was amazingly buggy, and lead to different entertaining bugs all 3 times my friends and I tried to play through it (only to end up at a game breaking cutscene bug). Expect scripted NPC pilots to randomly crash your osprey into a mountain, or to dump you out too high off the ground and kill you.
I'll second Brutal Doom. It's explosive gory fun as either solo or co-op. Alternatively, there's a bunch of mods for doom that also play rather well and can be co-oped, including some with RPG elements, tho they're all gonna be pretty fast-paced regardless. There's also other games of the era, like Heretic, Hexen 1 and 2, Duke Nukem 3d, Blood, Shadow Warrior, that allow you to play through the campaigns with other people.
Also seconding Magicka. The co-op's kind of a mutual hindrance thing where more players mostly mean greater chances of doing something wrong, but it's fun if you don't mind entertainment by way of your friends accidentally exploding you or sending you flying off a cliff. Was prone to major connection issues last I tried it, tho.
Trine 2 has a co-op campaign for up to 3 players. Tho whoever gets the wizard could basically solo the game.
Stardew Valley's a more "relaxing" sort of thing where you mostly just work to get upgrades and marry waifus/husbandos. There is a world with pre-written events that happen with time and all, so it's sort of a campaign, I guess?