Small speculation involving Perospero and Marco who we saw entering Onigashima together in the latest chapter.
Now I could be reading too much into this since we know Perospero was heading towards Onigashima and Marco had left for a bit to check out a suspicious shadow.
Hence it could very well be possible that Marco spotted Perospero at sea and either chased him to Onigashima or decided to team up for the moment.
My main problems with this are the following however:
1. We saw Perospero wanting revenge on King, Straw Hat and Marco, so I doubt they would ally themselves that quickly.
2. If Marco was chasing Perospero then how did he arive at the Front Gate? Marco could basically attack him all the way from where he was at sea.
This set me thinking, since when I first read about Marco seeing a suspicious shadow at sea I instantly thought of Moria because I believe he will show up in Wano because his back-story is linked to Kaido.
We however saw Moria not in Wano (as of yet) but on Beehive Island to find Absalom because he feared that his ability was taken by the Blackbeard Pirates. This turned out to be correct and it is Shiryu of the Rain that now has the Clear-Clear Fruit.
After rereading this chapter I came across this panel where Blackbeard invites Moria to come sail under his flag:
We also get to see Moria, Shiryu and Devon when Blackbeard talks about Wano and Big Mom
So basically my idea goes like this, what if it isn’t Marco that we are seeing but instead its Perospero and Devon? We found out that Devon has a mythical Zoan fruit that allows her to change her appearance.
It might even be possible, if Devon is pretending to be Marco there, that Shiryu is at the front gate aswell, since he can turn invisible now.