Honestly, i think this message is less about 2 specific people and more the "surprising" matchups that will happen as people get closer to One Piece.
An example is BB and Law. It already was surprising, and happening. We could have Fuji vs Weevil, or Shanks attacking SHF members, or Kid finding and attacking the burned scarred man. Or the Revo's assaulting the Marines or WG. Maybe Dragon vs Garp??
That is the "free for all" Oda is specifying here, including future fights on Egghead.