he silhouette is that of Shanks.
-Oda stated that 2021 will be the year of this character, this is already a gigantic clue for what is he, since Shanks's movements are totally mysterious and we would have an incredible 2021 with the red-haired present in the war;
-The fact that Shanks is in the reverie denotes an unpredictability when it comes to his movements, we saw him secretly entering the gorosei room where they have a mysterious conversation;
-From spoilers, one can understand that in one of the lines of reasoning, there are two unknown people in the treasure room, since we are told about having a person who does not look like the silhouette of 1004, so the logic could be that not only Shanks but Toki is also in that place. Toki may have traveled to the future in search of pirates who would help her with the conflicts in Wano, what would be the best pirates she knew if not the old gang of Gol D. Roger? Where she sailed for some time and befriended the crew of this gang, Toki may have gone looking for Rayleigh after the events of Wano (since she already knew Roger was dead), but soon she remembered Shanks, and may have gone looking for him, Shanks knowing his former captain would also do the same, he accepted the request for help and mysteriously arrived in Wano with Toki quickly (just as he arrived quickly in marineford shortly after being in Wano).
-Shanks in the war could balance the alliance side, since Kaidou and Big Mom are monsters and now that Kaidou is in hybrid form, the chances of the dome top alliance winning are slim, but Shanks can arrive and fight alongside Luffy to defeat the 2 emperors.
I am not American, so forgive me for my English mistakes.