I'll give my vote for Tau as I find their visual aesthetic very unique in the 40K universe.

Well, given the proposed War Council system etc. players are soldiers, tacticians and statesmen

My vote's on the Tau Empire. Nothing more badass than entering the battlefield with gun drones and an awesome battlesuit. We must make our own version of the 'War of Dakka'!

I'll be buying the game as soon as it has playable Tau*, Tyranids or Dark Eldar.
Imperial Guard maybe on sale.
Necrons maybe on sale, only if I can play as a wraith.

*XV25 Stealth Suit with Burst Cannon is a must!!!

Crisis battlesuits= getting my mech game play satisfied. Plus what it really boils down to is....do you like blue beamy death weapons or green beamy death weapons.

Necrons would be more fun, Tau would be more likely, the rest is opinion.

Tau would sate my desire for mechs(because I'm almost certain Knights are off the table).

What you don't wanna roll around as a floating robo crab/spider?

Oh come on, Necrons are so much more exotic and fun than Taus....

You got to ask yourself, what new stuff each race would bring to the game. What define Taus? Good guns. So that's it, we would just have blue humans in armor with accurate rifles which deal a lot of damage.... okay.
On the other hand, we have undead egyptians terminators who can ressurect themselves (which could bring new interesting mechanics) and who have new exotic weapons which could bring an entirely new gameplay to the game.