The base building is really good, with the gradual unlocking of new features. It's a great idea and a solid foundation for the endgame. However, i think what we need is more ways to fill up the productivity meter instead of the current expeditions that can quickly begin to feel repetitive and boring.
The maps could be improved upon to help solve this, by having them drop more often and adding new ones while also making the current maps more interesting. Make the boss dungeons more epic with stronger bosses, and have maps for multiple bosses at once. Have maps for 2 or 3 enemy density, and ones that grant the maximum amount of area modifiers for free, with some of them having pregenerated combinations designed to make an expedition feel more specific. Something like a map of those lambach enemies in their native tileset, that have bonus damage and resistance to rend, and where all champions/specialists/bosses have the blood ring and blood rain modifier. Also maps where the player is in permanent apocalypse form, or has only 1 extra life but also has larger bonuses to magic and gold find.
I think there are also enemy types present in the campaign but not in the expeditions, so add those in along with unused tilesets. You could also create expeditions that are not like a Diablo-style rift at all but are some sort of village defense mode or a boss rush mode, or maybe a limited-time expedition where the rewards scale with the amount of kills or points made during it.
There are a lot of things that could be done to make the city-building progression feel more fun and be a better endgame. Currently the expeditions are a little lacklustre, they need more variety.