I'm one of those people that thinks Oda set the Post Time Skip up to a mirror a lot of the Pre Time Skip.
Egghead is the Sabaody Arc of the New World to me, so, I think Gaban is the Rayleigh character.
In terms of the mystery character, I can't think of anyone who would fit other than Gaban who is actually strong enough to fight off CP0. Fujitora would make sense if the character wasn't hiding, and a lot of other potential suspects wouldn't be that polite.
Oda has cleverly made it so that each area the crew goes to slowly introduces us to Roger's legend/crewmates/allies. Romance Dawn starts with Roger's former apprentice Shanks inspiring Luffy; Loguetown gives us his legend from Smoker's eyes; Alabasta introduces Crocus; Skypiea shows that Roger went to the sky and knew of the Poneglyphs; Water 7/Enies Lobby gives us Tom's backstory with Roger and Roger's rival Garp; Sabaody gives us Rayleigh; Marineford gives us Roger's formal rival Whitebeard. I could keep going on.
Point is, Luffy has been slowly guided along by Roger's legacy this whole time. It makes sense for Scopper, Roger's other right hand man, to be the Rayleigh character in the New World.