Bounty hunters make sense, would explain why they been not around the big war 2years ago.
Unlike Gion and Chaton who been fanmade characters(which Oda love so much that he add them in his manga), they been probably planned before the timeskip(don´t forget Oda had in mind to kill Akainu and retire Sengoku from his position, so he always had the plan to add two new admirals) and been the strongest characters who been not in the navy/pirate fractions.
Speaking of Gion and Chaton, pretty sure both will be op too, like probably after Garp the strongest viceadmirals, low top tiers perhaps(if the Navy had them in mind for the Admiral position).
Unlike Gion and Chaton who been fanmade characters(which Oda love so much that he add them in his manga), they been probably planned before the timeskip(don´t forget Oda had in mind to kill Akainu and retire Sengoku from his position, so he always had the plan to add two new admirals) and been the strongest characters who been not in the navy/pirate fractions.
Speaking of Gion and Chaton, pretty sure both will be op too, like probably after Garp the strongest viceadmirals, low top tiers perhaps(if the Navy had them in mind for the Admiral position).