Tyranids and Necrons will be in first big expansion. I'm almost positive about it.
Actually, I wouldn't be super-shocked if one or the other, or both, formed the basis of a co-op expansion. As in, you can play against each other, or team up against this ridiculous tyranid deck! Or necron deck! Or whatever.
I think it's more likely that they'll be worked in as factions in the current 1v1 structure, though. Which I'd prefer anyway.