I think it will be them.
Because of the following:
- They can easily go away with less then 4 LL and a race pack, what the pre-order will be, has less then 4.
- A good part of their assets is already in the game with the normal dwarfs.
- They kind of fit in line withe the whole "defend against chaos" that goes for Kislev and Cathay.
- They are much smaller in terms of content compared to Chaos Dwarfs
- Without much stretching I think they could be put in Vortex as well as ME as well as WH3 map, depending for which part the pre-order race will be.
- If my lore-knowledge does not deceive me, they are not so big in gunpowder but much better in rune magic which could open up interesting possibilities such as Shard Dragons
- They are Dwarfs
- They might be generous enough to give their southern brethren a proper rune system if they have one too.
And because of Warhammer Old World.