Firstly, I'll point out that at least one of the Relic devs and Tranj, the THQ Rep, have previously stated that the DoW game engine can't do the Nids justice and hence GW won't let Relic put them in and Relic/THQ don't want to put them in if they can't do them well enough. While that doesn't rule them out completely I think it puts their chances of appearing as somewhat similar to that of the Squats.
Other possible races are:
- Witch Hunters (with Sisters of Battle)
- Daemonhunters
- Tyranids
- Squats
Personally, I think I'd rather see Witch Hunters out of all of those. It would fit quite well with the Soulstorm name as well, what with the Ecclesiastical nature of the SoBs. With the Throne of Judgement, Arco Flagellants, Penitent Engine and Preachers then I think they've got enough stylistic character to stand on their own and be different to everything we have already.