I would like to propose Christiania, Copenhagen for its socialist ideals. Its unique way of life manifests itself in the architecture as well as in local culture.
I'm refrained from proposing a Kibbutz in Israel, but if anyone wants to read more about it and consider to propose it - it's on the T list under Deganya.

Though a notable cause I am not sure if Christiania really is of outstanding universal value the infrastructure and architecture are all essentially a disused army camp, and I don't think the famous bikes would be enough to meet the work of human creative genius criteria. It would also create problems of sovereignty arising from Micronations, but hey this is a just for fun fantasy list so perhaps that is not important.

The Kibbutz idea sounds more reasonable. Though I must admit I don't have much knowledge of them and I wont pass off the quick glance at wikipedia as actual understanding but if someone does know a little more on the issue I would be interested to find out.

Socialist ideals don't really manifest themselves on the list, the planned industrial towns such as New Lanark, Saltaire, (Crespi D'Adda?) etc. show some of the early ideas that blossomed into the Socialist movement, but it is pretty tough to pin down political ideologies associated with particular sights. Independence Hall, Statue of Liberty and Monticello in the USA are all keenly associated with Liberalism, and there are a fair few Palaces in Europe associated with Monarchical Absolutism but political ideologies (independent of religious ideologies) don't seem to feature much.

You might be interested in this quote from the WHC in 1988 when they deferred New Lanark
" until an overall view of the sites where social doctrines of the contemporary world evolved in connection with the industrial or agricultural revolutions had been realized and a comparative study on the industrial sites of 19th C had been carried out by ICOMOS".

It did manage to make it back in but, since as long ago as 1979 (Edison National Historic site - deferred and not heard of again), there has been concern about criterion vi
"to be directly or tangibly associated with events or living traditions, with ideas, or with beliefs, with artistic and literary works of outstanding universal significance. (The Committee considers that this criterion should preferably be used in conjunction with other criteria) "

This explains why there are not many sites associated strongly with "Ideas" - they have to justify themselves in more tangible ways first!