- Castle Crashers is a classic multiplayer beat-em-up. Plays great on Proton (if that's important).
- Lost Castle is in a similar vein, though more roguelike.
- River City Girls (and sequels) is either great arcade brawler.
- Neurovoider
Great indie co-op games:
- Deep Rock Galactic. No split screen, but endlessly repayable with goals and targets in each mission. Also fun in solo.
- Caveblazers
- Guacamelee
- Tunche
Upcoming games to consider:
- Wizard of Legend 2