If BioWare is going to add new alien species to the setting I would prefer it if they weren't all rubber foreheaded aliens; a slight tweak of human culture thrown into a humanoid form.
I don't want to see all aliens we encounter to based around a single human concept; i.e. Krogans as the Warrior Race, Turians as the Imperial/Militaristic Race, Salarians as the Super Scientist Race, the Batarians as the Racist/Slaver Race, the Drell as the Assassin/Ninja/Nobel Fighter Race, etc.
I'd want to see aliens that are based around what makes them alien, what makes them uniquely different from humanity and how we perceive the universe. Aliens like:
Silicone Based Lifeforms/Rock Aliens
How would they react to us soft fleshy creatures? Would we be able to comprehend a sentient being that could be made of the very things that we consider building materials? Would they be able to comprehend us?
Unicellular Organisms/Amoeba Aliens/Blob Aliens
How would a space faring species comprised of only singular celled organisms react to creatures comprised of many millions of cells? Would they view us not unlike the Reapers; each a nation unto ourselves? Would they be able to comprehend something that doesn't reproduce asexually?
Insectoid Aliens
I know that the Rachni are an insectoid alien species, but like all non-humanoid aliens they got very limited screen time and development. How would an insect based society function? Would it be highly industrious? Would they be highly a voracious swarm, or a society that innately tried to form an equilibrium with their environment?
Plant-Based Aliens
Similar to the Rachni, we have already had a plant-based species in the form of the Thorian, but again like the Rachni, we are given very little insight into it, nor are we allowed to explore non-hostile interaction with it. How would a plant species, view the animal kingdom? What would they think of us if they came from a planet wherein they were the dominate lifeforms?
Purely Biologic Aliens
This could be seen as a subset of the Insectoid and Plant-Based aliens, but I feel that a concept like this warrants it's own category. What would a species that used entirely biological organisms for every aspect of their civilization be like? Everything from their spacecraft, to their weapons, to the homes they live in would be a living organism, perhaps even forming a symbiotic relationship with the commanding species. How would they react to aliens that build artificial constructs? If they exist in the Milky Way, would they have avoided the Reapers cycles, due to their total lack of technology?
These are just a few of the alien species I would like to see explored in depth in future Mass Effect titles, species that are based on a foundation of an alien concept rather than being based on human elements and then given alien aesthetics.