It seems a little late in the scheme of things to add Capitols, especially after the ammount of cash that would have been spent on all sides developing and deploying the Starbases. Still, it would be a way to shake up tactics even more, and ultimately they've left themselves no choice- The Capitol Ships are balanced to do what the whole list of Frigates/Cruisers can, but not the other way around, so a completely new Cruiser would mean a required new Capitol Ship if the game were to remain Balanced in this regard.
Note-This marks a lack of insight for the Vasari's Minelayer- It was the only unique new ship not mimicked in a Capitol, and the only race to do it.
Note-I see the lack of Long-Range weapons as made up for by the high survivability of the Capitols in Shields, Hull, and Armor.
All in all, i think the Diplomacy Expansion and all its new additions will lead to one thing- Unified TEC, Vasari, and Advent Against the mysterious enemy that's been chasing the Vasari, as the Third Expansion. Don't know how thy'd do it, but that would be an awsome way to round things off.
