TEB/Dogs of War is the best option for an early adopter bonus for the following reasons.
1. They are not expected at all (not part of the 16 main races) - so people would be excited about these new factions and not cry about them having a no DLC for DLC problem.
2. They connect game 1 and 2 - the factions in the east and the west would be playable - and the LL's could start in a boat off shore in either campaign.
3. It would offer a different playstyle instead of race to the vortex - in game 2 they could have an economy based victory condition because that is their goal to pillage the new world. They would also serve as a recuring annoyance to the main 4 races in game 2 as they come in their ships.
4. Some of the units could be used for a mercenary system in some of the factions for game 1 and maybe game 2.
5. The whole world feels more complete with less Empire clones, and it would give hope to others wanting Kislev to eventually be in the game.
6. It allows people who like playing human a chance to play one in the new world (and for people like me, a chance to smash humans with other races).
So CA if you are not planning already to make this the early-adopter bonus, I would recommend a change of plans, this would really complete the game and IMHO is the best way to promote game 2.
I want to point out that I am not a TEB/Dogs of War fan, I don't usually like playing human factions - in fact I haven't even played an Empire campaign. -But even then, I still think this is the best early-adopter bonus possible. Give a like if you agree.