Velo C.

I dont think there will be any actual physical confrontation between the two, and if there will be then it will be like in Marineford or how the fight between Luffy and Usopp went. No matter who Koby becomes, he will never be able to match against Luffy.

I think their fight will be more ideological, where two opposing ideals will clash

- Luffy's desire for freedom and chaos

Vs

- Koby's desire for order and justice.

I think Koby's existence is more as a hopeful ideal of the future of Marines. The world cannot exist without marines and just like there are good and bad pirates, there are good and bad marines.

I think Koby will play a secondary role to the plot, a reminder of whats good and just.

Unless Oda goes 180 and turns koby into the main antagonist after Teach's eventual defeat. But i dont see that happening