Cluckmech Oasis is a mix of action roguelike (Nuclear Throne style) with Tower defense that recently released a bunch of updates, including an online co-op mode. I have tried it for a few hours and it is a lot of fun, with different towers, difficulties and characters to try out (and, of course, being co-op makes it better).

I recommend it if you are into roguelikes and don't want something deeply complex. You can check out some gameplay here https://www.youtube.com/watch?v=21MQcFAyZ0o