Right know, we know
Aokiji->Kuzan
Akainu->Sakazuki
Kizaru->???
But Sengoku was the one who gave Spandine the power to do the Buster Call.
So who was Sengoku? He was an admiral, but who?
That gives us one slot for past Admirals.
Next, is Garp. Whitebeard, said that Garp, Sengoku and Roger remember the seas "the way it used to be"
This probably means Garp was the past Akainu, given the whole dog theme.
I think Sengoku was the past Aokiji, but I could be wrong. Reason behind this, is that history is going to repeat itself. Sengoku trailed Roger, as Aokiji is trailing Luffy.
This leaves the spot of Kizaru. Kizaru, is most likely the same one from the "Time of Roger". My speculation, is that while Roger was at sea, and Dragon was beginning the Revolution, Akainu, aka Garp, left Luffy/Ace with Kizaru, part of the Portgas family, and Ace's father. He isn't Dadan, because Kizaru would've heard of Enies Lobby, so Makino would have no reason to say "has dadan heard about this?!"
Kizaru is most likely a D. but not related to Luffy. I believe theres 3 branches of the D. Family, The Monkey side[Dragon, Garp, Luffy], The Roger side [including Blackbeard, and Saulo] and the Portgas side [Ace and his father]
Spending so much time with Ace and his father would give Luffy a reason to call Ace his brother, especially if it was from an incredibly young age.
So we have
Aokiji->Kuzan
Akainu-> Sakazuki
Kizaru-> Unknown Portgas, Father of Ace
20 years ago
Aokiji->Sengoku
Akainu->Garp
Kizaru->Unknown Portgas, Father of Ace.
Hopefully this speculation makes some sense… I think it got abit jumbled with the Ace/Kizaru thing, but hopefully someone can make some more sense of it.