There's no question the ARPG genre is heavily skewed towards fantasy, and there aren't that many sci-fi action RPGs that take advantage of the Diablo style. Other than Space Siege and maybe Hellgate: London, I can't think of anything better than the forgotten gem that is Harbinger.
Coming out at exactly the wrong time in gaming history, Harbinger was quickly forgotten as other titles crowded it out. Luckily, this underappreciated ARPG still holds up with a captivating story and Diablo-esque dungeons. That's even when you consider the shooting mechanics are a tad clunky and there are only three classes to choose from.
Sadly, Harbinger hasn't yet made it to Steam or GOG, but you can still find discs floating around on Amazon and eBay. It's also a good bet that some abandonware site has the files if you're willing to go that route.