Something that would increase replayability would be an equivalent of skirmish mode, as in the first halo wars, but with a campain-like feel to it, by having a map (planet or solar system) that would have different points where you can fight, each of which being a different battle, and the goal would be to conquer all of the map by beating your ennemy on each of these places, while protecting yours.
It would be like the galactic conquest mode from the old star wars battlefront, but with RTS battles, which I feel would really play great, and create more replayability than basic skirmish, since you would progress between battles.
What do you think?
remember that this is completely hypothetical and that I know the game is already coming out soon, thus making it hard to add more features before launch.