To help even out the Alliance in power play you could introduce a new power who took a small/medium sized chunk out of the alliance. The power could be somewhere in-between the Federation and Alliance as far as values go. They could be less happy go lucky and more practical minded. If you add Alliance ships this power could be the reason why you would add them.
The biggest reason to do this though is that the Alliance a large and unfair advantage over the other two super powers in that every player who is aligned has to pick or choose between multiple powers and the Alliance only has one which allows them to consolidate their player base unfairly. On top of this they have it the easiest as far as competing powers as it has basically been protected by the Federation powers for most of the games life.
The biggest reason to do this though is that the Alliance a large and unfair advantage over the other two super powers in that every player who is aligned has to pick or choose between multiple powers and the Alliance only has one which allows them to consolidate their player base unfairly. On top of this they have it the easiest as far as competing powers as it has basically been protected by the Federation powers for most of the games life.