I could see something like...
Resilience - FEAT 12
Req: Expert in Fort, Ref, Will
Benefits: Pick a save you are Expert in. Raise this save to Master
Being only able to take the feat once would be kind of important IMO.
2 Feats with the Rogue or Barbarian dedication let you do this as is. I took the rogue dedication on a monk character specifically so I could master all 3 saves.