Obviously a human protagonist makes sense because the trilogy was fairly based on humanity's growth in the galaxy(ME1, becoming a Spectre was the first step). Later in the story after defeating Sovereign(Alliance Fleet also played a major role in here), humans became so important that the Reapers perceived humans as a bigger threat than any other species which explains why ONLY humans were abducted by the Collectors in ME2(they were even building a Human Reaper on their base, HOLY ****) and Reaper's FIRST attack on Earth in ME3. Not only those but many other like curing the genophage, saving the rachni, peace between krogan-turian and geth-quarian. All this shows us the capability of humans and the impact of humanity over the galaxy. These accomplishments became possible through Commander Shepard, a human that we play as in the trilogy and of course, with the help of his/her alien crew as well.
Keeping that in mind... If ME:A is going to have even a little connection with the decisions that we chose in trilogy(definitely there is a connection IMO) then all of this would make sense on playing as a human again. Otherwise, a human protagonist just because we are humans lol. I also like the idea of playing as another species in single-player but that will change my view of Mass Effect universe from story perspective. So yeah, human protagonist makes more sense to me.
Maybe it is possible actually.
Something similar in ME:A with the side quests or maybe even main quest(s) related would be nice to make us all happy.