There are a lot of Couch CooP Games with only one copy and Gamesystem needed- but mostly indie games. Some friends and I get together from time to time and have a great time.

We switched to element (matrix-protocol) a while ago. Until now it worked fine for us - without any real problems. It already got a native voice/video-call implementation. But i heard that selfhosting isnt that smooth

https://element.io/de

Besides all the already mentionend games: Shadow Complex (2009) was a good game back then. Nothing special without any real story. But it was fun to play