I personally think that as of now (Spirit, episode 8), this is exactly what Unalaq is trying to do.
Unalaq seems to know a lot about the Spirit world. Most probably enough to know the significance of the portals, and what is hidden between their openings in the spirit world. I doubt that he considers them as just a bridge to the spirit world or a possible quick way to get from the North pole to the South (to better keep a unified Water Nation).
It is no coincidence that he is attempting to get both portals open around the time of the harmonic convergence. If he can get both portals open, he may be able to free Vaatu, in hopes of bonding with him. Though Vaatu's distaste for humans may just lead to him destroying Unalaq after being freed. Then again, Unalaq seems to have great control over spirits (he may even have induced the chaotic spirits to attack the South to strenghthen his political position), and may be confident about controlling Vaatu so that they can merge. Once they merge, he may be able to destroy the Avatar, and possibly be in a position to rule the world.
Unalaq was also seen coming out of the Southern portal in Spirit, Episode 6 (Sting). This may indicate an ongoing collaboration between him and Vaatu. (Update: As of S02E08, this part is confirmed)
However, this "Dark Avatar" may not have all four bending abilities, just a lot of power. Wan got his bending from the lionturtles, bonding with Raava just gave him the ability to use them all at once.
EDIT: As of S02E12, I was correct in judging Unalaq's intentions! Yay!