Let's be honest.
Reality is that probably this is just Pero.
But I ain't gonna lie I will be disappointed if this is only Pero.
We need more Big bois at Wano and fucking Ryokugyu already.
It has to be more than that though. Oda wouldn't have made Pero climb the fall only for Marco to find him and ruin that moment. This is definitely misdirection, kind of like the mystery person and Izo, whom I think is with the Guardians heading to the front of the island. As for who the shadow could be....
- Weevil. I doubt this because it would be oddly coincidental for Weevil to even know that Marco came to Wano and followed suit so easily after being chased by Marines
- Former Roger/WB pirate (Vista, Scopper, etc). I also doubt too this because they all could have just been together on Nekos ship (aka, I believe the mystery person was Scopper on Neko's ship). Plus, Marco leaving seems like he'd encounter an enemy, not yet another ally.
- The Marines/Ryokugyu. I also doubt this because they all explicitly say they see too busy and must deal with some other fallout from the reverie (whatever BB is going after; Pluton). Though, I can see Sword Members, aka Koby and maybe Hancock if he partnered with her. Its been a week since he attack encountered her I believe, though I'm not sure thats enough time to arrive at Wano)
- Former Rocks pirates: Shiki or someone else. This i can believe if they managed to hear the news about BM/Kaido teaming up. Perhaps Shiki caught the Mama Canter with his power and brought it back up :p
Idk, there's a ton of possibilities with this arc. So much mystery on who is supposed to show up, when, and how this arc is even supposed to play out. People dislike Wano, but I feel like all of these factors make it the most unpredictable arc thus far.