• 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