Re: "You cannot claim this city because you do not own some of the content used to create it"
Sorry, but there is no city I can't reclaim
When a city is created it list all DLC's of the creator in the city setup.
If you can't reclaim that city tou are missing one of the DLC's. It doesn't matter if the city has been payed or not
There are a number of DLC's, for some we had to pay (like the amusement park), others were given to all or at pre-ordering/packedge deals (like plumbop par or launch Arcology) or were available for a limited time period (Progressive insurance). Also there a a few promotinal DLC's like Crest, or Red Cross. These DLC's aren't available tot every one, it depends in what country you live or if you managed to score a coupon (Crest)
Beside those DLC's, there are 5 country dependend DLC's.
Media Markt: Germany
Telia: Sweden
Play: Poland
Metro: France
Micromania: France
These DLC's can only be obtained when you live in the country as specified, or you will to use a proxy.
(I don't know if they are still available)
In short, there are a lot more DLC's out there. If you can't reclaim a city, your are missing a DLC