Torchlight 2
It'll run well on just about anything (both hardware and OS) and its co-op multiplayer is quite fun.
Also regularly on sale for 5 or 10 USD.
Overcooked 2 when its on sale
Don’t starve together is one of my favorites that works on like every platform
Pretty much anything valve released on the source engine. TF2, half life 2, portal 2, either of the left 4 dead games, all run on Mac and have co-op modes.
With a bit of fiddling (i.e. installing the Windows version of Steam with Wine). I think they're all still 32-bit and Apple has dropped 32-bit support in recent macOS releases. Maybe that'll change since they're making a 64-bit Linux version of Team Fortress 2, but I won't hold too much hope.
Oh my bad, I didn't realize they weren't keeping on top of apple's architecture changes
Borderlands 3. I've never tried it in multiplayer, and it certainly isn't as well written as 2, but it's fun and it has cross play.
I second this, borderlands 3 story felt flat for me but the gameplay is so fun for me
One of the main problems with BL1 and 2 in multiplayer was that level disparity really sucked. If a mate had like a few levels more or less than you, that was it. So you needed to have a strictly MP character and a separate for single player, otherwise someone was having no fun.
Did they fix that for 3? Somehow, I doubt it.
Factorio. The best value for money game i have ever bought. It is never on sale, so no need to wait.
The factory must grow! But remeber that the factory can not grow if the engineer do not eat, sleep and care for themself and their family!
The “We Were Here” series.
Second Life RP. Star Trek Online
I would look into For the King
Stardew Valley, maybe? There is both a Mac OS and a PC version.
I don't know about Mac, but Deep Rock Galactic is the best co-op game I've ever played.
Factorio is great if that's your kind of game.
Recently me and my friends are playing game called Crawl. It co-op in some sense... One player is the hero, and all other players control ghosts who summon monsters and set traps. When current hero dies, ghost who killed him becames new hero etc.
A gaming community free from the hype and oversaturation of current releases, catering to gamers who wait at least 12 months after release to play a game. Whether it's price, waiting for bugs/issues to be patched, DLC to be released, don't meet the system requirements, or just haven't had the time to keep up with the latest releases.
^(placeholder)^