This is a tricky one, RPG and co-op generally don’t mix. Larian games have attempted that a few times, though they’re a deep dive. I’d recommend Original Sin 2 from their selections.

Then there’s hack&slashes, I played Grim Dawn with co-op once, but that’s more of a damage competition than something you really need to think about to progress.

Cassette Beasts added co-op recently, but unfortunately I haven’t had time to try

Then there are games like Borderlands, Dying Light or Dead Island which are far more action-y but do have levelling up and skills to unlock