Yes in "rational" circumstances Mecca would have to be on the list. But somehow I can't see Saudi Arabia reacting well to a group of ICOMOS bureaucrats questioning their management plan, buffer zones and "historical integrity"!! And in fact of course Mecca has undergone incredible and many would think, unsuitable, development very close to the Ka'bah. Saudi Arabia has wrestled for years with its conscience on far less significant heritage within its borders and has a record of almost wilful destruction of some of its physical heritage. Wahhabism fears the idolatry it sees as inherent in such objects. It will be interesting this summer to see if it gets any of its 3 relatively recently T listed sites inscribed. But I am hopeful I will get at least 1 addition to my visited list as the "modernisers" appear at the moment to have the upper hand on this matter against the issuers of Fatwa 16626.
However, for our purposes which are to identify the most significant non-inscribed heritage I support this suggestion!