I think those would all be positive changes. Right now the city enchantments don't feel very effective on just one cast. It's probably made worse by the silly amounts of mana that the AI can get. I don't know if it's a bug or not, but in the post-game review, Oracle Ceresa was shown as having mana that would literally spike up into the hundreds of millions (I was playing on Easy).
It would also be cool to have some defensive or passively offensive enchantment spells for cities.
By defensive I mean: An enchantment that prevents your city from being targeted by hostile enchantments, but perhaps has a high upkeep cost.
By passively offensive I mean: An enchantment that prevents the enchantment from sticking to your city, and does something bad to the caster, but gets expended in the process. Like a hostile contingent dispel, or spell mine.