There is a reason, why cities give max resource level 12 - that way they roughly produce twice as many resources as two villages (which is their counterpart).
Cities are intended as alternative to two villages and are currently quite fine from infrastructural balance. They give resources from 2 villages with much less effort, but less culture points IF you party in every village's townhall. So they're a very solid alternative for less active players. (Water ditch sucks especially with bunkermeta though, gotta mention that somewhere.)
Doubling the max level of each building for cities is probably the most overpowered idea that I have read in the forums at all so far. And I bet you did not think that through at all.
- Doubling max resource field level would basically make every city another capital
- Warehouse level 40 would cost 28.4 million resources (for 40th level alone) and have a capacity of 5.8 million resources (plus minus something).
- Barracks level 40 would produce ~8.22 times as many troops as a barracks level 40
- Academy level 40 most useful building xD
- Palace level 40 could train 7 chiefs
I could go on for a while, but I guess my point is clear. It just doesn't make sense at all. City's intention is an alternative to two villages and not to be a necessity to complete some buildings.
Honourable mention: You can build great barracks / stable only in cities.