I know this isn't on the tentative list yet, but I'm in Manila right now so I figured I would try to visit what I could.

Does anyone know of what sites in Manila might be part of this bid? Everything I've read about the proposed site talks about it in general terms and hasn't mentioned which sites would be part of the bid.

I too have failed to find anything specific on Manila - but i would have thought that the Philippines would have used any such nomination as a chance to place the whole of/much of Intramuros on the List. It would seem that the name of the site would "require" that Manila was included? But the other end at Acapulco provides a similar problem - it is a bit "thin" on good quality historical remains as well - just yet another "Spanish fort" of which the List already has rather too many!
There is already a memorial to the Galleon trade at the Plaza Mexico inside Intramuros - have you seen it? https://en.wikipedia.org/wiki/Manila_galleon#/media/File:Manilajf9742_16.JPG

But it would seem that the Philippines would go for other locations too - e.g Ruined shipyards in Sorsogon for instance. The author of this article is employed at the National Museum in Manila and provides her e-mail address so you could e-mail her for more info on the possible shape of a nomination?
http://www.themua.org/collections/files/original/34a74c76efdb951655b9bde1213812dc.pdf

And the nearby location of the first mass in the Philippines? How much "tangible" remains might be a problem however. http://rhescobal.tripod.com/losgaleonesbagatao/

This conference on the Trade held in 2015 ( http://nhcp.gov.ph/1729-2/ )mentions the Shipyards again and also the port of Palapag - a fair trip from Manila however! But Manila itself doesn't deem to figure - I guess its inclusion if at all would be symbolic and a way of getting another inscirbed site for the Capital.
https://en.wikipedia.org/wiki/Palapag,_Northern_Samar

Note also the "Underwater Archaeology" aspects of a possible Nomination which could even take in sites in Pacific Island sites countries such as the Marianas and Guam?

I would assume that some/most of Intramuros would be included. If nothing else, there is a place called Plaza de Mexico inside Intramuros which I would think would have to be included, but I haven't actually read anything confirming it one way or the other.

As I posted above!??