If you fancy a shooter then check out Deep Rock Galactic. You shoot bugs and mine minerals in randomly generated caverns with a team of three others with lots of terrain destruction and laughs.

It’s cheap, it runs on anything, the community is fantastic and the devs are great too.

It’s good enough I’ve put 50+ hours into it running at 640x480 at 20fps.

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Foxhole is a grand strategy war game with a huge logistics layer. You control an individual soldier and can fight, mine for resources, build trench systems, deploy poison gas, and get shot repeatedly by snipers. The game relies heavily on multiplayer coordination, with large clans driving logistics to gather resources in order to build tanks, artillery, bunker systems, and more. At the same time players will be pushing active fronts to try to conquer territory. It takes a few hours to get the hang of all the systems but if you want to be part of a big, persistent war it's a fun time.

I was going to say MyHordes but then you mentioned "massive learning curve."

I mean, its not so much a massive learning curve so much as a, if you mess up, you just die slowly in a day or two.

Civ6 is fun solo or multiplayer, although it's pretty heavy and can be a little grindy so it's not the most social game.

I think a lot of us will be playing Victoria 3 when it comes out, but it's gonna be heavy as well.

I'm super big on old school arena FPSes, so I will always recommend Quake Live, Sauerbraten, or UT2004.

Another one I will add is Mobius Front '83, which is a TBS that is challenging without being as heavy as my other suggestions. I haven't played it on multiplayer yet, but I think I'm gonna gift it to a friend of mine and give it a go soon cause I've been enjoying the campaign.

Battle For Wesnoth is an open source turn-based strategy game with multiplayer that will prolly run on anything. It seemed pretty good, I only played it once tho

Sven Coop is good shit albeit an FPS. It's a free coop mod of HL1, you can play through the entire half-life campaign in coop, and there's like fuckin hundreds of other coop maps people have made in like any genre, you got puzzle maps, you got escape room shit, you got mario party maps, you have a faithful recreation of the first dungeon of Ocarina of Time. You name it and someone's probably made a hl1 engine remake of it

War Selection is an RTS online battle royale, i dunno how low-spec friendly it is and i only ever played it for like five minutes

Not free, but Dominions and Conquest of Elysium are pretty fun coop games in the grand-ish strategy genre. Dominions is more of a traditional grand strategy game with nation management, conquest of elysium is more of a lower scale band of heroes type thing by the same devs. I highly, highly recommend pirating them unless you're strapped/they're on sale tho, as they are definitely not worth the like 40 bucks they ask for imo (multiplayer worked fine on a pirated copy). Good stuff tho, tons of different factions with varied playstyles. I won a game by playing as a undead army that kept on spamming a global rapid aging spell that didn't affect my skeletons, so that was pretty rad. edit: kinda a big learning curve tho now that i think about it, the UI was absolute ass

There's tons of online/co-op text adventure games on the internet called MUDs, AI Dungeon allows for multiplayer text adventure stuff, but I think that feature has a paywall and if anyone puts a horny command into the chat once the AI will start nosediving into a thirsthole (which can be hilarious). Space Station 13 is essentially a text-based game with minor graphical elements and can be a blast, but the learning curve is astronomical and some of the communities can be pretty shitty

I would say age of empires but uhhh there is a learning curve