Personally, I'm 100% sure that Larian will be brought in by the gnomes.
Even now, there are gnome characters (AND svirfneblins) in the game. I think we can play EA for them. In any case, it would be very strange not to introduce them.
It seems to me that the lack of half-orcs and dragonborn is due to the lack of a model for them. The dragon face is difficult to animate, and the half-orcs... Well, apparently the model for them has not been created yet. It seems that Blurg (Orog in Out of the Abyss) became a hobgoblin because of this.
Yah, dragonborn especially are going to be complicated to add in. You can't quite mo-cap a dragonborn IRL to get the expressions so it'll need to be modelled and programmed by hand. So I definitely get why they wouldn't have dragonborn for early access. It is a lot more work. I am hoping we'll have them by the end though. They're the only real "beast race" of the core races. It'd be a shame to have all these elf variants but not a single option for beast race lovers.
Half-orc I am sure will be in. They're close enough to human that you can easily use mo-cap like with the rest of the races.
Well, since the githzerai and githyanki are very similar in physical terms, Larian should have no problem creating their model.
It will be much more difficult to give the correct NPC reaction and dialogue options for the character. Imagine what a githyanki patrol can do when they see a githzerai.
True. Lae'zel is also a problem, but you can pretty easily have her overlook your Githzerai nature due to the presence of Mind Flayers. This is pretty much the only situation in which the two races are more inclined to work together than kill each other.
That said a lot of the Githyanki reactions would apply to Githzerai. It's not like the common person in Faerun can tell the difference at a glance. Githzerai would just need a common "I am not that kind of Gith" line for the most part.
Of course playing a Githyanki has a LOT of unique Githyanki dialogue options and I'd expect a similar range of options for Githzerai. So it'd still be a lot of work with dialogue in that regard. Probably still not as much work as adding, say, Assimar or Goliaths though.