In the same way as the The Architectural Work of Le Corbusier, the Architectural work of G.Eiffel has been already approached? Would it be a serious idea or not in your opinion? It could cover several countries.
You will have seen the Connection for WHS inscribed as "Multiple works of a Single Architect
So - Palladio, Vauban, Horta, Gaudi, Le Corbusier and FL Wright.
Leaving aside the issue of whether they all really "justify" being placed on a World Heritage pedestal, I guess that what they have in common is an innovative, coherent and developing style/technology which influenced the wider world and/or (Horta?) reflected an era/historical period, rather than merely being a collection of works which happened to be created by a single person. Some of the Nominations, of course, had problems in coming up with a subset of the oeuvres of the "great man" which satisfactorily demonstrated that!
What attributes/features would you cite as being Eiffel's particular "gift(s)" to the world of engineering and which works could do that?