Well see, the main problem I have with bringing back the citadel races is this: They'd spoil the reapers for everyone.
I mean, yeah the black boxes kind of did that already but actually having survivors of the reaper onslaught be around would make it completely undeniable. There will be no covering it up by the political element because there are too many witnesses.

Not to mention the old races would no doubt attempt to rally the new races against the comming reaper threat. (because that was the very mission Javik (the last prothean) and his project set out to do: conserve the prothean empire in a few thousand individuals so they could emerge and found their empire again in order to rally the new races against the reapers.)

I mean, it's all fine and dandy to have them be there but the implications for the RP would be huge.

Just take my race, the Kimo for example. At the moment they are using the reapers as boogiemen to keep their own population in line. But the moment they hear that the Turians (a council race and the basis of the vast majority of their own technology) are still around and warning everyone the reapers are very real and actually comming... well, they'd shift into a mode where they will be actually preparing for the reapers. Like moving to capture the citadel and actually building the crucible.

Like I said the implications of that are vast and I fear people just didn't put enough thought in that before they said yes to this.