Strowhats visiting Sphinx island makes a lot of sense, I actually wanted to make a thread about this, but since you already did, then I'll put my thoughts here.
Luffy should visit his brother's grave to drink the sake cup, it has been set up. Plus, Luffy and marco never had any dialogue in Wano, which is strange, now it seems that Marco had already left Wano before the strowhats did. Which means that we might have a conversation between Marco and Luffy in Sphinx island. And last but not least, Weevil's plot line can be resolved in Sphinx island. Weevil will eventually find out where Marco is, and will come to demand his inheritance. Luffy being there helps to drive the plot, on top of that there has been a slight hint from Oda that Weevil might join Luffy's crew.