What?
Although I applaud what you are attempting to do, I think that you are going to find a very difficult time as even if you manage to find a NPC duchy, which is going to be like finding a needle in a haystack as even dukes are having problems finding land for their duchies on NA-E, all of the duchies are all in kingdoms, so you will have to answer to the kingdom laws of wherever you settle. I don't think that there will be an area for an independent city.
Countess of Tarnham
County Tarnham, Rhynelands Duchy, Vornair. Luna Server (NA-E)
It can all be negotiated in advance. If the mayor is forced to be annexed by a noble before gaining the duchy through game play, I think they'd agree to keep the area independent. There are always advantages to independence.