If you haven't tried them, APB: Reloaded and Defiance are shooter MMO types that you can get into fairly easily. When you're first starting out in APB:R, most of the missions you run will be super easy because people don't use cover, but you'll get gold level threat quickly and be challenged or aimbotted to death. Aside from that, it's an open-world GTA MMO with an extreme level of customization, pretty fun. Driving is server-sided, get used to that lagtime and you're good. Defiance is fun but extremely slow after a certain point.
MOBA games are to-taste, if the community is too salty we understand. I have to take breaks between weeks where I play DotA 2.
Blacklight: Retribution is okay, they've done stuff recently I'm not thrilled with. There's a free Tom Clancy Ghost Recon type game that's pretty fun, can't remember the exact name. It's free though, and fairly tactical.
Realm of the Mad God is a neat MMO-thing on steam for free, and you could look at Endless Sky if you were keen on it.
Mabinogi is a Korean MMO you can definitely sink time into. It's running on an impossibly old engine and network optimization was done for a different country (so I'm told), so it won't run great, but it's got some neat systems you can give all of your time and money.
Dragon Nest is a free game that combines... I suppose it's a 3D fast-paced brawler, but it's online and it's mostly dungeon crawling/RPG stuff. The combat is a MMO/fighting game hybrid, heavy on the latter.
Both Mabinogi and Dragon Nest are Nexon games, so enjoy that P2W/Cosmetic sinkhole if you're familiar with them.
Spiral Knights is doing okay now too, you could give that a try.
I think Dungeon Defenders 2 is a free multiplayer team PvP pseudo-TD now.
Magicka: Wizard Wars should also be free.
Could give Trove a shot too.