I always loved Campaigns in RTS games. I'd go as far to say that in RTS I prefer the campaign while in TBS I usually prefer the sandbox.
But then again, I don't think I've seen a real campaign in a BTS before, I think the closest thing to come was in a mod- the Fall From Heaven scenarios.
I think most TBS campaigns (including Gal Civ) was like "here you go, an extra part of the story has been revealed, an extra weapon is unlocked, and here we have to take over the galaxy again (or take over a new galaxy).
No ... I think the Campaign should not be about taking over the same (or similar) maps over and over again.
Instead, start small, with each campaign the map gets a little bigger and you have to take over a little more (consistent/progressive).
Equally, some Campaigns should not involve cities in any way, and just be questing. You can still get gildar from defeating foes and selling loot however, so you can also use that gold to buy Mercenaries and potions.
--> "Each level should 'add' to the previous levels, as opposed to having to start from scratch, imho"
--> Some stages control of the cities should be taken away from you. Heck, maybe there is a civil war and most of the cities now belong to an enemy, or something.
--> Maybe you get deposed from the throne and you now have to SOLO against your former kingdom, relying only on mercenaries (and perhaps a few loyal soldiers).
I really think any campaign in the Fallen Enchantress universe should be very lore focused and story focused, with good story telling elements such as a strong hook, a sense of actual progression, possibly some twists, and a low point near the end (Dramatic turnaround by the enemy, followed by a necessary counter turnaround by the heroes)
Maybe the gates to the shadow realm are unleashed, maybe the hero needs to enlist the aid of either the dragons or the exalted ones (gets to choose between ancient heroes or dragons to help him).
Most 'conquer and expand' scenarios should take place on the same map, with previous progression saved for the next stage.
Primarily quest based (solo) expeditions can take place in entirely different maps however ... as a travel to other worlds, other continents, etc. (into the wildlands of the Calebethon, etc)
If you have to solo against your old kingdom however, you should be taken back to the same map your progress is on, and your own units, techs, and cities will now be working against you.