What are you talking about
Koby is already the next Garp. He's already known as a hero of the marines, literally no other marine bar garp is called a "hero". Why is Koby called a hero? Because he did something that helped Teach beat Wang Zhi... almost the exact same thing Garp did.
Also, you can't use Koby's desire to be an admiral to downplay him. You are treating admiral as if its some sort of power level that is inherently below pirate king, which it isn't. Koby not only said he wanted to be admiral, he also said he wanted to capture Luffy. Then in marineford, he literally tried to fight Luffy.
The fact that he isn't Luffy's level is besides the point(because Luffy has no true rivals), he occupies a similar role to Garp in Luffy's story much more than any other character does.
You can bring up smoker, but Koby objectively has better hype than smoker. Haki training from garp, acknowledged by Teach, hero of the marines. Once Koby gets a DF or haki awakening, he will be one of the strongest marines.