Githzerai, because I think they are at least ten times cooler, than githyanki, if you would play planescape torment you would know...

Although I doubt, that they would go crazy adding a lot of monster races, as I think they are a lot of work. Maybe with some shapeshifting or modded items, you can play around with different avatars at battles...
I am quite happy already, that they have drow,
if they would have originally planned Githzerai as playable race next to Githyanki I would be already very-very happy for the game, hoping to get my first physical copy of my collector s edition, as soon as possible if they have.

I have played Planescape Torment but I have also played Neverwinter Nights 2 which I think tells us more about the Githzerai and the Githyanki also you end up getting the sword of Gith the funder of the Githyanki but the Githzerai and the Githyanki are not really monsters I was thinking on the lines of Kobolds or Goblins
Githzerai vs Githyanki is like comparing, the revolution of Gandhi vs the bloody French Revolution. Both is for the good cause, but one of them is more spiritual, therefore more philosophical.
I would love to play my main character as a Githzerai monk or someone in touch with the teaching of Zerthimon.

Regarding the real monster companions, I am happy to see some kind of Lycanthrope, Yuan-ti pureblood, a burning man, or a flying skull xD...
Anyway these are not important for me, maybe in future expansions or future dlcs...

Goblins would be a trouble, as I think we will meet a lot of goblins as enemies and they are better as enemies.