Actually Tau are more suited to fight orks than ig : cause they don't really have some decent artillery. Personally I will play Tau as soon as they make their way in, IF they are well implemented : would be very easy to make an awful Tau faction.

Necrons. But it doesn't really matter, as long as they get implemented at some point I'm happy. The great thing about 40k is the diversity. Which means I'm supportive of any additional race getting added. The more the better.

Adeptus Soritas. Cause lack of female orcs, Speesh Marines...while thinking of it...hot female Slaanesh worshipping Females?humm...Good geared and ready to go female Imperial Guards? The Eldar will have some wimmens...
Ah well. I go with Adeptus Soritas. Their stuff always looks so fancy, like it.

Also tau are too kawwai at this point of development, i want the dark Universe of 40k for now,where there is only war and not only pew pew ;-) Bloody Chainswords,Screaming,from ranged to close combat,eye to eye,when you know what i mean

Imperial Guard is rather like must have: Imagine a Comissar running around executing CoD players with silly names attracted to that faction for "having comitted heresy". Alternate: "I told you to aim properly n00b!" *BLAM*
jokes aside being a standard imperial joe standing against all the grimdark horrors with your standard issue lasrifle and balls of adamantite, winning atop of all that would probably feel equivelant to "Highest difficulty completed" of any hard shooter out there, thats if the idea executed properly ofcourse.
Considering i still couldnt get used to Newcrons and Khornate Knights gonna say Dark Eldar because you know that you too want to use your hair as a weapon. Awesome.
Sororitas would come in handy too for the imperials, besides they are pyro heavy faction and i like flamers(or atleast have been in DoW)