I would like to protect my proposal for famous architects. It is similar in character to the one about famous painters and certainly adds to our understanding of the sites. There aren't enough such artists who have 3 or more whs and yet I suppose we can learn about the history of architecture through such a connection of architects who only have one site on the list. Palladio indeed has his own category, but it is not dedicated to his work solely but to others who imitated his style as well.
Maybe this proposal can be rescued if we stress what exactly the intended monuments are (if they are not the entire site already).
I don't remember my list exactly but I try:
Last Supper - Bramante
Acropolis - Phydeas
Gaudi
Hortas
Tugendhat - Van der Mies
Palau de la Musica and San Pau
Vatican - San Petro, Bernini
Rome - Musei Capitolini, Michaelangelo
Vicenza - Villa Rotonda and other, Palladio
Wuerzburg - Balthasar Neumann
Vienna - Otto Wagner (Postsparkasse, Secession), Loos (Loos House, American Bar)

These are all important architects in the development of Western art. I don't remember if I suggested others as well, but I think these do earn a connection.

Waltar Gropius - Bauhaus
Filippo Brunaleschi - Dome of the Duomo Florence
Oscar Niemeyer - Brasillia and Arts Centre in Le Havre
Francesco Borromini - Rome (Quatro Fontane +)
John Vanbourgh - Blenheim Palace
Christopher Wren - Greenwich
Inigo Jones - Greenwich
Karl Freidrich Schinkel - Potsdam & Museuminsel

Additons and tweaks to ones above
Donato Bramante + Vatican, Urbino and Rome
Giovanni Lorenzo Bernini + Rome
Balthasar Neuman + Palace at Bruhl
Lluís Domènech i Montaner - Palau de la Musica Catalana & Hospital San Pau
Ludwig Mies van Der Rohe - Tugendhat Villa

Gustave Eiffel - Eifel Tower Paris + (but already covered by another connection)

these are all known architechs of the 20th century but whether they can be classified as famous is a different matter: Jorn Utzon (Sydney opera House) Luis Barragann (Casa Barragann) August Perret (Le Havre) Max Berg (Centennial Hall Wroclaw)

Do you know which building(s) in Potsdam?

As far as my research goes, Phydeas 'only' has made sculptures for the Acropolis. I have not seen him named as architect of any of the buildings there.

I accept the correction. Maybe we could include his name under Statues.

Ah sorry Els I was just rattling through them and was being very vague sorry.

In Potsdam it is:
Babelsburg Palace, Charlottenhof Palace, Pomonatempel, Roman Baths and St Nikolai's Chrurch (though I don't think this is covered by the nominated area) this also rules out Mendelson's Einstein Tower

In Museuminsel it is the Altes Museum.

Also should have added Nickolas Hawksmore in relation to Blenheim Palace