The only confirmed playable factions are Chaos and the Imperium of Man. Here is the page in which you may speculate about factions.
101: By Fidei Defensor Edit
1) Eldar and Orks are clearly in the game. The former is yet to be featured but I'd be very supprised to not see them in the game. Eldar have been a core faction for years.
3) We have 3 featured races. I would be fair to assume that the faction number is even. DMO may have, at minimum, four races and at maximum, 6.
Based on the tabletop grand campaigns like Armageddon or the Eye to Terror (which for balancing purposes group races together) Order and Destruction lists look like this:
Because the Imperium of Man factions are united as one in DMO setting their are only 3 Order factions. For Order to have 4 factions one faction must switch sides. This is unlikely but in the event that it is 4 v 4. The most probable candidates are Dark Eldar and Necrons. Dark Eldar have supported Tau and Eldar in the past. Necrons have supported Imperial and Eldar forces behind the scenes in wars against Chaos and Tau in wars against Tyranids.