What I'd like to see them do is just vary up the character models a bit.
Using ME3's enemy types as an example, imagine we had the reaper troops as they currently are, but there were a handful of character model variations for each type, particularly the bipedal ones. Like three different models for Cannibals that deploy at random. One is shorter and fatter, another is taller and more stout, etc.
Just like people are all people, but can be shaped differently.
I'm not sure how feasible this is or is not from a development standpoint, but it doesn't seem like it would be that difficult to have a handful of alternate character models for each enemy type. They'd still be readily identifiable from a pure gameplay standpoint, but it would add some visual variety.
As it stands now, when you see a Cannibal, they all look identical. Same for every other enemy type. It makes a bit more sense for the Cerberus units because, from a lore standpoint, they're literally assembly line soldiers. But even then, they're not clones. Surely some would be taller and some would be shorter, etc.
The point is, even with a static set of enemy types, it would be nice if they could introduce a bit of variety through alternate character models.