I could see coby as a commodore to be honest. Right now coby is a captain however we know he has gotten to a point where he has at least 1 type of haki if not 2. We have seen how captains fought in part one of the manga, they are not that tough. Even with what little we know of coby today I would argue he is well stronger than what most of the captains in part 1 ever showed. If he has armor haki then he would actually be at least a match for part 1 smoker (whose only redeeming feature was to be a logia). Coby is nowhere near admiral level today but he will certainly get there relatively quickly (which is not too many years from now).
I hope he won't become Admiral at all. That's like Konohamaru becoming Hokage (sorry for reference to different manga but it seemed fitting). He is a good friend to Luffy and he admires him. He wants to be as strong as him but as long as Luffy is alive that won't happen. For me either Smoker is Luffy's Garp or there won't be a "Luffy's Garp" at all.
I already thought Coby's improvement was too much so far - the Rokushiki skills and now Haki... Compared to his original strength he is getting stronger at a faster pace than Luffy imo. If he actually becomes Admiral and rivals Luffy then he would probably be the character with the most potential in the whole One Piece universe - and he doesn't deserve that. He was a whiny little bitch and now he is strong - that's fine. But hopefully never admiral.
How do you know that it will exists marines in the future?
I certify Coby as a formidable combatant able to withstand any kind of situation in future, though. He's guided by the same willpower that spoke Roger, even Shanks acknowledge him for that.
I believe just right after Luffy becomes pirate king and Zoro becomes greatest swordsman, Koby will become an admiral; yes an admiral not the chief admiral, this is a big different in term of position and power too.
By eating mera mera only helps him achieve his dream easier, otherwise he has to work extra hard in order to full fill his personal goal.