Alright, I'm looking for good Coop games to play with my 11 year old. He's pretty hooked on Fortnite, which I don't really enjoy. We've got an XBox one, and 2 computers, so it doesn't have to be split screen, it can be local LAN PC to PC, or crossplay between the XBox and PC, or split screen. Doesn't have to be GOG only either.

Could probably be any genre - I got him a little into Hollow Knight today when none of his friends were available to play Fortnite, and he enjoyed that.

We've got Trine, and Trine 2, which we played when he was a lot younger, we may give that a try again. I've never tried Serious Sam in coop, but I've got it, and apparently it is fun. The kids have played played a bit of Overcooked 2 together, but that doesn't really scratch the Fortnite itch for him.

Any recommendations?

Terraria?

Also... co-op - otherwise it looks weird!

Mmm it could be Streets of Rage 4, River City Girls, Divinity Original Sin Enhanced Edition, but it could be a little complex at the beginning, Blazing Chrome (very difficult), Stardew Valley, I think Sundered has up to 4 co op split screen, Cuphead, Swat 4, JazzJack Rabbit 2, Yooka Layli, etc.

Castle Crashers is a pretty fun co-op game. I've finished it with my friend a couple of times. It's available on both Xbox and PC. It has local and online co-op, but I don't know about crossplay.

That game is 100% not appropriate for kids in the least bit. It has tons of adult content.

If a free MMO is not out of the question, and a cartoon "motif" is acceptable, I'd suggest taking a look at ToonTown Rewritten.

It's a lot of fun, and there is no adult content. The "violence" is stylized (no one "dies" - NPC's or players, they just "go sad" for awhile). Did I mention, it's free?

Two players can coop-solo (just those two players alone) all the way thru the main questlines; and can stay together for the various optional boss fights (that typically have teams of eight). (I know this because I dual-boxed [well, two instances on a single box] all the way thru the main quests).

Here's a short preview video (from the main site linked above):

What's this game all about?

HTH

Rayman Origins/Legends?

I would recommend Trine series.
It's really good if you have kids / someone to play with. Puzzles may get a little hard mid to end game, but it's fun to solve them.

Since you both played 1 and 2, try the 4.
3º is not that good compared with the previous ones, but the 4º it's a good come back to the series.

EDF 4.1 or EDF 5
They are the embodiment of co op fun. Your kid will love it, you will love it. You're welcome.

Yeah maybe :/

You may also try:

Grounded:
https://www.youtube.com/watch?v=ciVgq5xO5G0

+1
If you use Steam, only one account needs to have the game, there's the option "remote play together" wich uses internet to play one on each pc (me thinks it uses streaming).

On my experience (check this thread for some family oriented fun games) fun games work the best to keep kids hooked and forget the usual online games for a while. Here are some that worked for me with a single kid:

Minecraft : classic game, kids love it and I really like it as well. We tend to prefer exploration over sandbox construction and while is very buggy, Orespwan mod has provided LOADS of fun. Did you ever thought in Toruk Macto (Avatar's giant bird) as a pet, flight the bird around and fight dinossaurs?

Broforce Nice 2d platform co-op game that connects very well to us, grumpy "old" timers, due the various 80's and 90's action movie characters. Kids love to provoke explosions so I can lost a life in the process...

Brawlhalla Free to play 2d platform fighting game, easy to play and quite hard to master (kind of Smash clone). Online free-for-all or 2v2 can get a little chaotic but is very fun (and frustating if you take the game seriouly and play competitive). 2v2 is the most fun and require a little co-op strategy to succeed.

Flatout 2 ,Destruction derby Grid 2/Autosport Demolition mode and similar can be fun as well. I never managed to get any of the kids into racing games, except those and:Trackmania (free from steam and here, there's link in the text for the free version )

Mario Kart type, those are fun to anyone, you can setup a Nintendo 64 or Gamecube/Wii emulator or play any of the clones ranging from the free and open-source Super Tux Kart, Garfield Kart ( currently free on indiegala ), F1 Race Stars, Sonic Racing, Grip or Re-Volt

Rocket League Football (soccer) + Hot wheels/micro machines, loads of fun. Super Tux Kart has a similar mode.

I don't know what kids like these days, but I think those might work:

Overcooked
Metal Slug X
Stardew Valley
Terraria
Think of The Children

Gunfire Reborn maybe? Torchlight 2?

Or Portal 2, if he can appreciate using his brain for a change. ;)

Battleblock Theatre, The Adventure Pals, Rayman Origins/Legends or A Hat in Time if you both enjoy platformers.

Octodad for something goofy. Stikbold! A Dodgeball Adventure might be fun, too, vs or co-op.

The lego games are nice for a coop session with the kids and some of them are available here ( batman, star wars...)