I'll use some old Rock Maple I have lying about for the instrument. I'll make a slightly shorter scale length to be kind to my crippled fingers. The finger board will probably be Rosewood, possibly Purple Heart or Formica.
Everything else will be as basic as is possible. BUT!
I am tempted to route from the back side a channel that will coincide with the bridge location. I'll rout it down so the top of the banjo is abour 2 MM thick at that point. Then I'll fasten in a transducer for a bridge pickuo and embed it in Epoxy. (Will it work? Don'y know. Thats why its an experiment.) But I will have at least one magnetic pick up near the finger board. And maybe just for shits and giggles I'll build in a spare Sabine tuner I have kicking about.