He is already a Vice Admiral so a DF power up + a potential new Jitte could put him around the admiral tier. He will be strong enough to give our MC a great fight but Luffy will always be at the top when we reach the end of the story.
People have been hyping Koby as the new Garp of this generation due to Oda's mistake of calling him a rear admiral. At the rank of captain, there is no way you can take down a Warlord and even less a Yonku. I do believe that he will become an Admiral but only during the epilogue when years have passed.
Imho, Smoker fits more the profile of becoming the new Garp:
- He has chased and almost captured the future PK on multiple occasions.
- Smoker knows Luffy so much that he can literally guess his next move such as going to Rainbase, and Raijin island
- They have this love and Hate relationship that Roger and Garp had. Koby is too friendly and fanboyish while Smokey is tsundere.
- Hina noticed that Smoker was actually happy when the Straw Hats successfully escaped Alabasta... He just doesn't like to show it.
- He knew right away that it was Luffy and co who saved FI from Hody's rebellion.
- The PH accident made them closer that even his G-5 subordinates ( look like pirates) are starting to love the main characters.
- Smoker and Garp were Vice Admirals when Luffy and Roger started their final run.
- Luffy and Smoker allied against a common enemy in PH and in Alabasta too.
- The WG covered up Luffy and Roger heroism and gave all the credit to Smoker and Garp.
- The series is about parallels and inherited will. Smoker even got a similar facial scar !