Well, most (if not all) DS action RPGs are Japanese and in Japan they aren't into the whole loot gathering thing and put their effort into a good story and battle system instead (or dungeon design if it's more of a Zelda-type game). So if loot is a must for you, I think your only options are roguelikes. I personally don't like those type of games so I can't recommend you any good ones, but if any type of action RPG will do I'd recommend Lunar Knights. It's more of an action/adventure than an action RPG, though.
Phantasy Star Zero might be something to keep your eyes on since the PSO series has always been very Diablo-esque.