Do the same thing as orks, Guardsmen or conscripts can be F2P, vet's can be the payed people ... ta daa!
I just came back to say... Can Lost & the Damned be added to this poll? They should be in this, would sell it outright for me if that was the case.
I'd say Sisters of Battle for a change (though I don't plan on playing them,) but either the Tau or Necron would be awesome. Tau makes more sense as a playable faction while Necron could be a very nice alternatives to the Tyranids being the only population balancer.
How would playable Necron even work as a playable race, fluff wise? Seems like it would be the same problem with playable Tyranids in the sense that they are supposed to operate collectively without any individual thoughts (not counting the Necron lords). I'd vote for Necrons being a new NPC threat in a future expansion. The way to balance playable IG would be to give every player a squad of friendly NPCs which he can customize as if they were his equipment. Each member he chooses to bring in his squad would affect what abilities the player can use. So if you have, for example, four slots in your squad, you can bring maybe a vox operator to get some support ability, an ogryn for tanking, a sniper for high DPS, and a guardsman with a krak launcher for anti-vehicle. You could swap these allies out at base based on your requisition for "veteran" allies in much the same way a space marine player could upgrade his chainsword or whatever. This would keep the race in line with the lore but also remove the balance problems that would otherwise arise when trying to make a fair fight between an IG character against a superhuman or xenos.