Help a guy out!

http://store.steampowered.com/app/258970/ try gaunlet, don't know too much but it looks cool and is co-op

It is for PC, right? I dunno, there isn't much avaiable that suits all of that criteria... Sometimes, when I wanna play a coop game with one of my brothers, we just download a PS2 game and emulate it in our pc connected to the TV haha.
Now, being more serious... Did you see landmark? It seems like a very good game... I had a beta invite, but I can't play it because of my GT 610. Google it :)

'Mount Your Friends'

Entirely unrelated to ALL your criteria, and basically just multiplayer QWOP with floppy wieners. It's infused with some kind of magic that means if you play it with your significant other you don't stop laughing and time goes by faster. Highly recommended.

How about the newest Borderlands game? The PreSequel will be out on the 14th

Why don't you get games where she can design the dungeons and deathtraps to kill you in? Like maybe http://store.steampowered.com/app/250560/ or similar. Also, there's this thing http://store.steampowered.com/app/290080/ out now. Both should meet your criteria even though they're early access.

there is Gary's Mod which is fun

or the new borderlands if your pc's can handle it

Divinity Original Sin! I haven't had a chance to try the co-op though.

I know a few you would like. Magicite, Don't Starve, Castle Crashers, Trove, Unturned, Unepic?

How about Magicka?

Robocraft is free, and you can craft spaceships and vehicles to do battle with! Squad together and blast other people and their creations. It's a hoot. Shooting instead of looting though.

If you've never heard about the Neverwinter Nights series, you may want to check those two titles out. You can get them on gog. ^_^

Anal. Always the gift of anal.

Life is Feudal
RUST
Space Engineers

Don't Starve,How to Survive,Portal 2,Diablo 3,The Incredible Adventures of Van Helsing 1-2,Torchlight 2,The Lord of the Rings War in the North ... Some don't meet your criterias but all are fun in co-op.

Also I played THE KING OF FIGHTERS XIII STEAM EDITION with my GF recently, she loved it.

If nothing works, try anal.

Bedroom.

Divinity Original Sin, unique feeling and fun when you sometimes disagree with ingame-partner...Rock-Paper-Scissor ^^

PC only... oh... I would have suggest some some toys.. er.. I mean non pc games...