Rocket League - I don’t really like sports games or driving games, but somehow I love Rocket League. It’s just pure fun. None of the DLC is compulsory; some of the new cars control ever so slightly differently, but largely it’s all cosmetic.
The Payday 2 GOTY is probably worth picking up, especially if you have friends that also have the game. It’s slightly less appealing solo or with pubbers, but can still be a lot of fun. I played the shit out of it up until the microtransactions hit, and will likely be getting into it again at some point now that they’re gone again. The GOTY edition includes all the DLC that I consider to be pretty much necessary.
Divinity: OS is a super solid game. I’ve never managed to finish it because I keep trying 2-player co-op with a bunch of flakes, but one of these days I plan to start a solo game and do a full playthrough.
Hammerfight is one of the few games that I pretty much ALWAYS keep installed to fire up when I don’t know what I want to play, and it’s dirt cheap right now.