On the one hand it would be great if you could work first in a big model of a city, then create different proposals, make a border of these proposals (for example town districts) and then create a new model of one of these proposals. In my opinion that would be way to handle a model easier if you don't need the whole context of the city any more.
And on the other hand it woukld be practiacally if you can also add/merge a smaller model in a big model.
Kind Regards,
Eric