So far, Koby has two major ranks that he needs to achieve before admiral. Rear admiral and vice admiral. Even if he doesn't become admiral by EoS, he will surely be vice admiral this as vice admiral is garp's rank and the bar to be vice admiral is very low.
Coincidentally, Koby has two mentors with these ranks. rear admiral drake and vice admiral garp. I think that through their deaths, Koby will eventually reach and surpass these ranks.
If Drake dies, I think Koby will eat his devil fruit. The admirals are animal themed and these animals are from the zodiac. In alabasta smoker used a move called "white snake" where he turned his arm into a "snake" made of smoke. This parallels the admirals' animal attacks, and indicates that Smoker will be the snake admiral.
Pheasant, dog, and monkey are right by each other on the zodiac. Ox and tiger are right by each other on the zodiac. If Koby eats drake's fruit, he will surely be the dragon admiral as he would have the dragon dragon fruit model: allosaurus. The dragon is right by the snake on the zodiac