I'd rather see the Thargoid thing done really well, rather than having a whole host of sentient factions. There has been plenty of decent sci-fi with one (or less) sentient alien race in it. Alien, Blade Runner, Battlestar Galactica, Firefly, Dune (I believe) and more.
Generally media which uses multiple races ends up making each one hugely stereotyped, which I often find a bit lame. Mass Effect was guilty of that.
Would love to see plenty of non-sentient alien life though, and it would be really cool to have creatures that are just on the cusp. Kind of like the alien equivalent of chimpanzees, or maybe even slightly cleverer.
imagine them being curious about your ship, maybe steal a little part then your landing gear has trouble retracting or whatever
anyway i agree about fewer sentient species being better.
if there are everywhere, finding new ones will quiclky get old, especially if they're all humanoid like.
I'd rather have one specie like in 2001/2010 with the monolyths or something as mysterious and somewhere else another specie far less advanced rather than a star-trek like galaxy full of quasi-humans with a very similar technology level for the vast majority. "Ok you found the 152th species, so what? it just look like half of the others".
And precisely because it's a game, it's much harder to make many species interesting and distinct enough than just a few.
with thargoids and perhaps another one, you can take the time to do it right.
though the galaxy is big enough to make a few more than just 2 or 3, but that may come later in time, the unexplored region is likely to stay quite big for a while.