I don't know many that are like PSO either. But since you list Diablo, I can name a few Diablo-likes.
Obvious suggestion, Torchlight 2. It's Diablo but looks better and runs on a potato. Fun multiplayer. For me the dungeon grind is ...well, a grind, but if loot games are your thing, it does that. Torchlight 3 supposedly sucked, and Torchlight 1 is single player only.
Remnant From The Ashes 1 or 2. More Dark Souls than PSO, and requires a beefy PC, but good stuff if your into mixing shooting and melee.
Heroes of Ruin on 3DS. Only played the demo, and it would require some shenanigans to get it online, but might be what you want.
Spiral Knights. No idea if this is even still running, but it was a simple MMO with a dungeon crawl focus and a cute art style. Likely the closest to PSO in theme and tone.
And finally, maybe something like Dynasty Warriors if you like a more hack and slash game instead of a RPG. No idea which are good though. They've done sci-fi versions over the years too.