I'll be honest with Joshua's confidence at a low, probably Wilder would win if the fight were held today. However, Joshua is the better man with the FAR better legacy, so I am picking him to rise to the occasion if it ever does happen.
I can imagine Joshua being all smiles, street brother turned nice guy act, then looking like a deer in the headlights and all gunshy for a few rounds before Wilder lands a few big ones, wobbling and dropping him. For a number of reasons, Joshua is an athlete in terms of strength and condition, he's not a fighter though.