To say a game is "like Diablo" is almost always to make an invidious comparison, since few studios are capable of the level of polish that Blizzard brings to its work, and for indie developers, working at much lower budgets, it's hard to do something that quite compares
Rather than imitating Diablo, it's better to do what Depths of Peril does -- build on the tropes of Diablo to create something quite different, and quite interesting in its own right.
At first glance, Depths of Peril is indeed a Diablo-esque RPG. You control a single character of the usual range of classes (warrior, mage, cleric, rogue); you go out on quests to surrounding areas, slaying lots of monsters, earning XP and money that you can use to improve stats and equipment. Combat is fast, Diablo rather than Final Fantasy, and there's the same huge range of variety in equipment and magic items.
But -- layered atop this are AI opponents that remind me of the opponents in Railroad Tycoon. You control a "covenant," which consists of you and up to 5 other characters you recruit (and incidentally, you can take one along with you when you go adventuring, which is extremely useful). Each of the other covenants -- up to 5 of them -- is busy adventuring and building up their own heroes' stats and equipment while you are.
Essentially, you are a sort of "fantasy hero corporation," competing against other fantasy hero corporations. Instead of laying track and carrying cargo, you're out their slaying evil monsters and stealing their treasure for the greater good of mankind, but so are your competitors, and in the end, the town isn't big for the two (or six) of you.
The real enemy isn't the orcs or whatnot, but those nasty bastards on the other side of town; and you win by eventually becoming powerful enough that you can go on a raid, and wipe them out. Or, for that matter, build an alliance with other covenants, and earn an alliance win by taking out your mutual competitors.
Now this is all rather--odd, in its own way; but it has some real strengths as a gameplay dynamic. For one thing, when you complete a linear RPG like Diablo, you're done; you could replay it, but it'll basically be the same game all over again. In Depths of Peril, you may well find that after five hours of gameplay, having explored only a small fraction of the world, you've succeeded in obliterating the opposing covenants. And by cranking up the difficulty level (which is measured on several independently choosable scales, by the way, including opposing AI aggressiveness, starting levels, and so on) you can wind up playing a much different game. In short, while it may not have the infinite replayability of a Civilization, it has vastly more replayability than a typical RPG.
Depths of Peril has enough connection to familiar game styles that it's not hard to pick up and play, but also enough difference, in its need for diplomacy and conflict with competing AIs, that it provides, in all, an innovative and different experience from any other game. And that's something that, at least in my opinion, is worthy of admiration.