This question was raised in my mind the second I read 2D2Y Manga..
Luffy's going to have to fight BlackBeard who probably has an Army behind him atm..
BlackBeard is probably going to not be much of a threat..in my opinion..
On account Luffy clearly dominates him in Physical Ability..(shown on there last encounter)
Plus BlackBeard is strong. But not as Physically Strong as Luffy..never has been..nor probably will be..
The only idea I have that BlackBear will be a problem is the fact he can create Black Holes <_<
But thats my thought..what are your ideas?