For me its.
1. Kaido's past. From recent events though it seems like Kaido was nothing that special during rocks, but after BM giving him the DF he was able to get stronger. Also about his actual origins as to why he was not refered like other characters but as a thing by BM etc. I am still...
Speculations since I am bored.
1. Location of 4th Red Phoneglyph.
Current known locations of all 3.
This was the case when Roger was travelling too.
It seems Roger already knows about 4th Road Phoneglyphs location or atleast had the Idea(which is correct too).