Well anything else is going to take a lot of work. So if Im going to do that I'd want something better tha the 5 turn thibe.
suggestion:
How about you decide to raze at any time, but razing immobilizes your army for a period of time, based on a factor of number of units in army/city level * number of improvements in city. Catapults allow for 1 turn insta-raze no matter what. Rabble for the purposes of razing is just as effective as elite units. You'd also gain gold based on amount of improvements.
Might be a bit difficult for the AI, but a fairer system.
Really, what I'd settle for and be ok with it just having humans and AI play by same rules instead of giving the AI a cheat.
On the UI rules- even if it's just a 1% difference, like in GC2 where I'm pretty sure now AI to AI tech trading was on a special rule, I can really feel it in the areas the AI and human play different (it annoys me when I see the AI do its quests via diceroll in FE for example, even though I understand the logic as to why it is this way)