We had this conversation a long time ago when that episode dropped, but here's the list of candidates (and why none of them are it):
-Scopper Gaban: For some reason, people think Scooper Gaban is the one with a burn mark. No reason, just that since he was on Roger's crew, they might know something. Kid had the Wano and WCI Red Phoneglyphs, so he should only go look for Zou's and the fourth one that went missing, so I doubt this person who is dettached from this mad race to the finish should know something about the location of the last one. May give info on the one from Zou, but that's kinda it.
-Kuzan and Sabo: People defaulted to Kuzan and Sabo when this came out, but Sabo has no reason to hold that information or care about it and if he had it, he would disclose it ONLY with Luffy. Kuzan makes equally as much sense since he is cooperating with BlackBeard, so if he knew something, he'd tell him. Also, people believe he's a double agent for the Navy still, so I heavily doubt he'd let pirates get closer to the One Piece, even when undercover.
-Dragon: Just as Sabo, no reason for having the information about the Red Phoneglyphs and also people mistakenly thought it was a burn mark on his face when it's clearly a tattoo.
And after mentioning all the people who definetely aren't it, the most likely candidate discussed was... Actually no one we know. Most sensible people believed that the burn mark character is not someone that has been introduced to us yet and it may be a new character who, either is a fishman (burn mark could refer to the mark given to slaves and to the Sun Pirates, which form the Sun Mark. Technically a mark made by a burn, so it counts) or an undisclosed person with ties to the Underworld, since Kid has been shown trying to buy slaves at the Auction House in Sabaody and attending Caesar's Shinokuni presentation.
Also, heads up, if you wanted a clear answer/discussion about this, you should've gone to the manga channels cuz this channel is for informed, long paragraph-form theories substantiated with evidence and not a one-line open question thing.