I think 6v6 Dominion actually makes more sense than in any other game mode.
-No limitation on resources given that Dominion already has the highest passive experience and gold gains.
-Fast paced action that keeps all 6 players in the action all the time.
-Throwing an extra player into the mix will create even bigger battles at top or possibly a different strategy of putting a duo lane at bot which improves the viability of certain champions.
-With more players, it might give more power to AOE/AP type champions as opposed to bruisers.
Not sure what other points I can think of right now but I think it really deserves a closer look at the very least. Our greatest shame is that the community holds little interest in dominion but I think Dominion really does have its own game mode fleshed out and adding another player feels like natural progression.
Remember to upvote this discussion at the top left corner if you support the idea :)