Where are the former admirals?
The former admirals are the ones who had the position of admiral during Roger era and now are retired or semi-retired from their duties. They were the next characters:
Current Sengoku as the the former Kizaru
If you compare Akainu and Aokiji with Kizaru you will see that Borsalino is more older than the other two and perhaps we was promoted earlier just at the same time when the current Sengoku became promoted to his current position of chief admiral.
Garp as the former Akainu
When Whitebeard talks about the strong people from the past he speaks about Garp and Sengoku in the marine side, it is possible that Garp was one of the admirals in Roger´s era.
Garp has the Inu theme around him all the time, his ship has a dog mask, he wears a dog mas and it is possible that with this them around him he was the formar Akainu.
Another point is "Garp as a hero of the marines", from the current info in One Piece it can be concluded that Garp got the title for cornering the Pirate King several times. I am sure that he was an admiral and his title "hero of the marines" came from his old position that he abandoned for a more relaxed duty as a vice-admiral.
It could be cool if young Garp will be based in Kitano Takeshi in a flashback of Roger era.
Dadan as the former Aokiji
We only know something about Dadan, he is the man who trained Ace and Luffy and at the same time a good friend of Garp.
It exist the remote possibility that Dadan is a close friend to Garp and Sengoku too making him the possible third former admiral who retired completely from the marines and took the responsability as the trainer of Ace and Luffy from a period of time.