They may have blue skin or scales, but many of the alien species in Mass Effect are essentially humans with a palette swap. Apparently, BioWare considered having a non-humanoid squadmate join Shepard, but discovered that it would have taken away from the rest of the game.
"All party members needed to use a humanoid skeleton," BioWare's Dusty Everman said. "If we'd ever tried to use a non-humanoid, the cost would have been huge. Instead, we took all the development effort that we could have put towards an odd squad mate and made a larger, more polished game."
Casey Hudson explained that any alien squadmates "had to function like the human characters." Primarily, they needed to "be able to carry a gun."
However, the Mass Effect universe is home to other, odder species. For example, one of the most beloved aspects of Mass Effect 3 was its Blasto easter egg. The galaxy's first Hanar spectre, many fans wouldn't mind seeing the next game in the franchise offer such a unique character a starring role.
"For Mass Effect we developed a set of additional aliens, that would mainly be featured in the Citadel," Casey told OXM. However, "each developed a bit of a following, so over the series we found ways to make them play more important roles."