As a thought, the money that they are making, and probably will continue to make, off selling DLCs may have a bigger picture. Perhaps they won't have any expansion packs, and the revenue they generate will come from DLCs. They could offer expanded features via DLC. It doesn't have to be reserved for just new leaders and scenarios. At the very least, I would hope that if they do offer and expansion pack
a) it either overlaps the DLCs, and it that case, those who have purchased the DLCs get a discount or
b) it is completely new and different content without the DLCs included, and everyone pays full price.
Either way, I think they owe it to gamers to devise a scheme that takes into account their loyal fan/consumer base and the discontentment some feel towards the DLC.