In my opinion, the aliens are what helps me to believe in the huge scope of the galaxy in Star Wars. It's one of the things I like in the PT over the OT. We see in the cantina (and later in Jabba's Palace) that there are many other species on this universe, and I find it very strange (from a narrative POV) that only humans formed the Rebel forces. Specially when the aliens are the ones being reprimanded. In RotJ we see a new species on the Alliance (Mon Calamari), but it's still lacking.

But answering to your question, I think Grievous is an example of an alien that could be replaced by an human. And for a dramatic stance, the droid army of the Separatists (although aliens would do, too).