Instead of the razing command we have for cities right now, how about we have three new options:

1) destroy city - this command just destroys the city and maintains fertility - perhaps -1 to fertiliy.

2) loot city - this command takes 2 turns per level of the city. You get gildar and research points (the random kind you get in the diplomacy screen) plus a combination of other resources depending on you faction traits and hero/sovereign skills.

3) Drain - this is a replacement for the raze command and the one that will net you a negative diplomace rating from other faction. You drain the magic from the land and destroy its fertility but you gain substantial mana from doing so - perhaps 50 per level of the city, takes 3 turns per level of the city. probably destroys the fertility of the surrounding 8 squares as well.