To get ideas for my bike, I oogle over pictures in old Harley books. Look at the items that were stock or Harley accessories from back in the day.
A Road King with hard bags takes it's influence from '60-'64 FLH Duo Glides and '65 and up Electra Glides. Quite a few of the items on those bikes are still available today.
I never really thought of the Road King Classic's leather bags as that nostalgic, but they take a nice nod to late '50's FLs.
There's plenty you can do, just look at lots of pictures and get ideas.