In regards to transgenderedness?
I presume that, in the mass effect universe, they can allready change the mind to fit the body or body to fit the mind, if one so desires, so they would be exceedingly rare and only exist, from discovering the mismatch til it's fixed.
Now crossdressers... that might an idea...
You know, this reminded me of a conversation I had a while back. Some context.
Paizo is a tabletop game publisher that produces the Pathfinder Roleplaying Game system; an alternative to Dungeons and Dragons. For each of their character classes, they have an "iconic" character that is meant to represent the ideal of that class and so when you see that character in artwork, you know what class is being shown. These iconics do have some degree of backstory and character to them, but not a whole lot.
About a year ago, they debuted a whole new batch of iconics for their Advanced Class Guide. The iconic shaman, Shardra, was a male to female transgendered dwarf, who was designed and written by Crystal Frasier, a Paizo employee who happens to be a transgender woman, herself.
It caught a lot of people's attention that Shardra's method of transitioning was rather similar to the real world method of hormone therapy, despite the fact that magic to switch genders instantly and perfectly was established to exist in Pathfinder for some time before Shardra's introduction. When Crystal was asked about this, she explained that she felt easy, magical fixes like that trivialized what trans individuals go through in the real world, and she didn't want to do that with Shardra's story.
That kind of thinking isn't uncommon in fiction. The reason AIDS, cancer, and other series diseases haven't been completely cured in the Marvel and DC universes, despite the super geniuses that can bend the laws of physics to their will who inhabit those worlds, is because the publishers don't want to trivialize serious issues in real life.
Point being, that if Bioware were to decide to include an explicitly transgender person in Mass Effect, it might behoove them not to make the transitioning process as simple as some fans think it would be with the advanced medical technology. In creating what some might assume is either a scientific eventuality or otherwise, a utopian future for their target audience, they could instead end up alienating the very people they hoped would identify with this character.