Relaxing multiplayer game's

You can check Albion Online i don't know how much content does it have on pve but it is super relaxing.

Warframe is multiplayer, no competition, just making builds for frames and farming a lot.

100% Orange Juice.
It's kinda works like a board game with most stuff determined by dice, so anyone with any common sense knows that basically anything can happen and it's thus completely un-competitive. Most people just play it to mess around and see what utter insanity will happen this time. Also has co-op mode and many seasonal events regularly.
Main negative is that it has buttloads of characters (and some cards) locked behind small cheap "DLC" which is kinda annoying if you're an achievement hunter (like me) but since none of them are overpowered it's not a big deal and they recently made it so there are two random characters free on a weekly rotation. (Most DLC characters are just gimmick-y and actually worse than the base game characters, so I guess I'd call it Pay-To-Troll-Pick. ^_^' )

Gunfire Reborn
a mix of binding of isaac & Borderlands with Co-Op
of course youre always starting over but you can skill your character a little and unlock more items through gameplay .

Im on No man's sky with a friend at the moment and it's very relaxing.
Raft is really good too

And maybe Green Hell but i don't try this one in coop for now

Meadow :)

Warframe and Path of Exile are my go-to free-to-play games with friends.
I can't think of any other MMOs but here are some good co-op choices...

Party-Based RPGs
Castle Crashers
Gauntlet Slayer Edition

Since you mentioned WoW...
Final Fantasy XIV
If you don't mind paying a monthly sub it is a game I always recommend when someone says MMO.
You can enjoy the free demo from the steam store page as long as you like and with friends.