So, I have been thinking:
One thing that kind of worries me is the fact that, after one conquers the city of Mondstadt, there isn't much left to oppose any credible resistance, except a city. For me, I basically only had the last battle Liyue left after that.
I comes in part with the fact that once you can get a pack that can beat a city, nothing in the game can oppose it : ennemies comes in groups of 3-4 max, vision holder are alone of don't move and bosses are also static. So, at worse, you can just get your killer pack and clean the map bit by bit while you quietly make a second one. I fear this tendency would only get stronger as more regions are added.
Which leads me to some thought to counter it :
- There could be a Teyvat threat gauge (like the national ones). It would rarely go down, but would only go up on big events (taking a city, making a full settlement, capturing the highest threat vision holder of a country…). Instead of releasing vision holders, it could make the resistance harder : for example, making 2 units spawn instead of 1, have all subsequent vision holder move with supporting units, or leads neutral nations to attack you first (right now, if you don't touch Liyue, you can conquer all of Mondstadt and they will just watch you). It could also trigger some mini-event, such as the reactivation of the ballista in Liyue if you haven't captured it yet.
- Having event that damages you directly. I can imagine traps/stratagems from the enemy that says that next turn, one of you units and all units in the same location take a lot of damages, but you don't know which unit (some corrupted girls could be sent to investigate and would give you the unit type/location/other. But only one girl can be sent, and she only bring one information). Or another one that would damage one unit in each location for 2-3 adjacent known locations.
- Having adverse effect in location like the withering zone in Sumeru, or the Tatarigami in Inazuma. They could inflict damages, or turn some units crazy. Or making the crossing to Inzuma very dangerous (a chance to have all your units killed during the crossing, reduced to HP halved if Beidou is on the ship)
- Giving the enemy some army (6-8 units) that move aggressively.
- Some wandering ennemies that could target your main base. They would not attack it, but if they reach it, they free one of you prisoner each, and she gets back on the enemy side.
- Having some "rebellons". If a colony is not colonized at 100%, there is a chance that wandering units can raise a army in it/near it to try and take it back.
These are just thought I have. Not all of there are good, or applicable. You probably have other plan, but I put it here just in case some of it can be of use.