It's essentially a mythological crossover. Say... get Norse, Egyptian, Greek, Celtic and old Japanese mythology in there? Offers a bunch of stuff, three are well-known, two are neat, and none of them offend anyone.
I suppose it's hard to pick a unifying villain. In light of that, I suggest that you have two campaigns (as well as normal hack-and-slash mode where you can mix and match as you see fit). In one, you play characters from Team Evil (Norse giants, Greek titans, Izanami, Unseelie Court, Set etc.) while in the other, you play Team Merely Morally Highly Questionable (Zeus, Thor, Zeus, any faerie ever, Zeus, etc.) Whichever you play, they get their own levels, and fights against members of the other team.
Now, where does the RPG aspect get justified? Well, whichever team you're playing, at the start of the game, they get de-powered by the other team. And their gear is lost. Said power is then absorbed by the opposition. This is why stuff like Cerberus is an adequate boss fight, the gods you fight are giant unstoppable juggernauts, and you have to run around regaining your power. Lower-level mythological beasts... okay, those are still mooks.
The talent trees represent regaining power and replacing it in some of your portfolios. For instance, as Apollo, you could invest into sun magic, archery or music (group buffs?). Maybe you could use your flaming chariot to run mooks over?
Equipment is thus explained too. You'd have to settle for lower-grade stuff at first, mind you. Ares might have to compromise and use, say, Achilles's sword or something? Maybe having one weapon/suit of armour and upgrading it over time would work better.
DLC ideas include Mayan/Incan, Native American and Slavic mythology. Lovecraftian if someone insists on it, but it wouldn't work that well.
Sorry if this isn't what you meant. Have a stupid wall of text and bad ideas that no one will read anyway.
Really. Can you explain to me how mashing square is better than diablo 2?
