At this point of the game nuke shields have become some kind of worthless. It is not possible to hold a city with big armies because it gets nuked. So I would propose that there should be 2 kinds of nuke shields. First one is the shield we now have. 250 geos, permanent shield, but destroyable.
Second there should be a shield that costs much more (5000-10000 geos) but can't be destroyed. Only thing is this shield should disappear if the owner of the city changes (like in previous rounds). This way you can really defend cities (e.g. resource cities) if you like.

no i dont think so.

with this kind nuke shield,resourse cities could be held for long time,
the game would be much more imbalance

no i dont think so.

with this kind nuke shield,resourse cities could be held for long time,
the game would be much more imbalance

Wrong. You forget that armies exist in this game. You dont HAVE to nuke every city you know. This way would actually promote city holding and city defending. The way it is right now encourages hitting and running, and penalizes city holders.

I actually don't agree with this because of the instant undestroyable shield. Back then there was slots to sorta make it a gamble.