I like the idea around Garp and Sengoku being former admirals, the less powerful of them now is Garp who took a more relaxed life and Sengoku… he doesn´t seem to be affected by the years like he is eternally young.
If Sengoku is a "Buddha" then he must have achieved a type of Haki different than the usual one and the "Conqueror King", this Haki not only increases the strenght but makes the person who has it able to do the following things:
-He takes the years more slowly then others.
-It is very hard to wound them
-Nearly Invicible to the illness.
The other person who could have this type of Haki is Shirohige, this help him to live all this time at the top and make the wound made by Roger something very remarkable.
A fight between Sengoku and Shirohige could be interesting, I can imagine the old man being like Nightmare Luffy but without time limit and with Haki, making a spectacular entrance and being only stopped by Sengoku.