I have been disappointing for a while now at the lack of incentive offered for attacking a city directly. What one gets is basically just basic res if one succeeds, and is nowhere near a reasonable reimbursement for loosing a large chunk of ones army (even with a small garrison a LvL 20 wall can cause serious grief for an attacking army)
The only real options for city warfare are either Blockading the city (only affects caravans and so on) or sieging. Sieging might be a great mechanic and all, but it enables one to inflict TOO MUCH damage on another person, who spent MONTHS working on that city every chance he/she got. The investment is too big and the loss is too easy to sustain, even with the restrictions on the siege encampment.
What i propose is a form of city occupation, where one can attack a city and "take over" the city for a while. Your army is parked in the city and enjoys the defense benefits from it's wall. Also you get 25% to 50% (may have to rely on research here to determine the level) of the various incomes from this city, as in basic res, tax income, mana and research (would be transferred to the city where the occupation army originates). If there was advanced res being created, 1 in 5 of these would be siezed upon completion by the occupying army (if he has caravans parked in the city with the army).
Crafting items would not be affected (too precious, would be squirreled away in underground vaults upon completion or something)
Now, in order to NOT make this too overpowered, this SUBJUGATION of a city would only succeed for 1 in 4 attacks (25% chance of it occurring) when employing this specific method of attack. Also, troop production in the subjugated city would still be allowed (underground resistance of a sort) and you'd be able to make troops in 2 ways, 1) the troops being produced are normal but take 3X as long, or 2) the troops take their normal time to produce but are only 1/3 as good as normal troops. Also, you can attack the subjugating army with these produced troops, and seeing as these troops are already inside the city and attacking, this would negate the wall defensive bonus for the players city troops only.
This stratagem would, in any case, only last the 15 days of any other "stay on this ground here" stratagem