This does not concern suggestions mentioned above, but currently upgrading village into a city kind of screws up your crop production in normal 6 or 7 crop villages. Getting instantly 500pop and total pop going around 1,3-1,4k does not actually allow much more troops to be produced. Resource production upgrade is really nice, but after all crop is the most valuable resource in the game. And concerning the scarcity of 9c and 15c sites on the map, it might be a good idea to allow a bit more crop production on other cities. This is a war game after all and currently crop is very limited supply and that limits players armies a lot.
Solution would be a new building, fishery, working like a grain mill or bakery and would enhance crop production 25% further. Fishery could need lvl 10 ditch to be able to build and maybe be placed on a ditch? Below a bit math on current situation and what it could be:
Current situation using 0% oasis villages and +25% crop boost:
6c village produces 2250 crop and with around 700 pop you can support ~1,5k troops
7c village produces 2625 crop with around 700 pop you can support ~1,9k troops.
6c city can produce 4219 crop -1350 pop = supported troops ~2,8k (1,3k more than village)
7c city can produce 4922 crop -1350 pop = supported troops ~3,5k (1,6k more than village)
6c city: 4922 crop - 1350 pop = supported troops ~3,5k (2k more than village)
7c city: 5742 crop - 1350 pop = supported troops ~4,3k (2,4k more than village)
How about effects on croppers?
9c city: 6328 crop - 1350 pop = 4,9k troops
15c city 10547 crop - 1350 pop = ~9,1k troops
9c city: 7383 crop - 1350 pop = ~6k troops
15c city: 13305 crop - 1350 pop = ~11,9k troops
So the improvements would not be huge, but it would counterbalance that pop boost nicely. Also I think it would make improving villages into cities a lot more appealing choice than it is now. If needed this building could be excluded in capital to limit crop production there.