I particularly like this idea, though I think we should include the Tentative list sites as that would limit our choices in some countries that have extensive/ excessive tentative lists.
I think the limit on not nominating from your home country is reasonable, though I would hope people would not get too partisan anyway, I could only think of a few sites in the UK that I think would be worthy of inscription
I am not sure if there was some other way to doing this or if I was supposed to start a new thread but anyway here is my own favourite, lifted almost directly from my profile:
Major Buildings of the Chicago School of Architecture (C i, ii, iv)
The Chicago School revolutionised architecture by essentially placing the 'skyscrapper' as key component of modern urban planning. Chicago is such an architecturally rich city and its main buildings have shaped many cities world wide. There are also several buildings through out the USA that could also be included in New York, St Louis, Buffalo etc. The main architects involved would be Louis Sullivan, Daniel Burnham, Dankmar Adler, William LeBaron Jenney, John Root and to an extent Frank Lloydd Wright but his major works are a sperate entity.
It seems odd that there are no 'skyscrapers' on the world heritage list as they are such an important part of Modern town planning, this seems especially odd as many other aspects of modern architecture have been included yet 'vertical architecture' is only represented by San Gimignano (Italy) and Shibam (Yemen) which date from the 13th and 16th centuries respectively.
If only to generate discussion, off the top of my head I was also thinking of: Amritsar(Ind), Ur(Iraq), Plzen Brewery (CzRep), Irish Round Towers, Olympic Park in Munich