I'd like something like Monster Hunter on PC. Or Witcher 3 with a co-op mode. Maybe something like Killing Floor but fantasy? I'm not really sure what I want but if you can think of anything that sort of meets those incredibly vague criteria I would be grateful. Basically I just want to hunt monsters with swords and stuff with a small group of friends. The only thing is we're all a bit sick of Diablo-clones so top-down ARPGs are a no-go.

How about Terraria?

...Though it does mix a little bit of sci-fi.

The perspective is the verboten top-down, but Baldur's Gate and Icewind Dale are quite mutiplayer-friendy. You can run with a couple NPCs if you don't have six live people to play all at once, or run with a smaller party.

Well, the Guild Wars series are not exactly like others MMO in some aspects so maybe it fits what you are looking for (You make a group in a town with any of your frineds and go on a mission with them. You don't see any other players while you're doing it, only in towns). And currently the 2nd is free, so you coult give it a try :)

Monster Hunter IS on PC, but it's an MMO.

You can Co-Op Skyrim.
http://www.nexusmods.com/skyrim/mods/67038/?