As the City sets are actually diversified, I believe most of the collecters have a certain preference like collecting fire engines series. police series, harbour series and so on.

Some of the members do have a couple of nice buildings as well.

Maybe interested members could contirbute their respective collections and setup a diorama for each series and by doing do, you will have a combined City series.

Maybe, those with Indiana Jones or even Batman could contibute their collection to be shown as a movie set on the City setup.

Like those HO train scale, you could have a train running around with even a castle (Kingdom series) as tourist landmark.

Btw, I'm concentrating purely on City construction series.

thanks cheng fei, just giving my 2 cents worth 

lego city series alone is impressive but if there's a combination of other series with the City, it will be awesome like having a Star Wars parade in the city with all the Star Wars Clone troopers or Stormtroopers minifigs marching with police escorts just like what they do in the US....wishful thinking but then, nothing is impossible with Lego