Having more alien, "non-humanoid" species to fight would interesting. But only having them exclusively?
I actually, like, hate the idea. A lot. Mass Effect is a cover-based shooter - if maybe that wasn't obvious. Now, the thing with cover-based shooters is: Enemies need to make use of cover. And shoot. I know right? So, to make use of cover and fire weapons effectively, enemies need to be humanoid. Sure, maybe a four-legged alien dog could use a huge boulder as cover, but it couldn't vault, or peek out and shoot back. Or shoot at all.
I don't know, maybe some people think cover is boring and just want to get close and punch everything (Vanguard, anyone?) Those people would be all for this idea. But I've always played as an Infiltrator, and being able to snipe, and use cover, and be tactical, are things I've cherished.