For fun, I thought it might be interesting to include an informal top missing sites that remain undiscovered and may even be mythical. When I say "mythical" I do not mean El Dorado or Atlantis, instead sites that have well-documented evidence of likely existence and at least sustained efforts of discovery. Considering tombs/tumuli, there have been significant efforts made to discover these sites below. There is also a myriad of lost cities that are believed to have existed.

Tomb of Alexander the Great

Tomb of Antony and Cleopatra

Tomb of Achilles

Tomb of Genghis Khan

Probably not world heritage material but I have these:
Flor de la Mar shipwreck
Amber Room (not the replica)

Burial site of Attila: https://www.thevintagenews.com/2018/04/19/grave-of-attila-the-hun/