These are some Indy ideas I had years ago and posted here, I think they would make for some good grounding for an Indy film with an older Indy:
Set 1961-Objective, the Philosopher's Stone--Same storyline with some tweaking as Iron Phoenix. Noir, maybe even some horror ala TOD, perhaps in the Lovecraftian vein. Following the format of TOD--Non Judeo-Christian supernatural force at work. [Additions - changed date from 1940s to 1961; perhaps use Prague and Eastern Europe as locales as well as the Atlantic ocean]
Set 1960- Objective, the Garden of Eden. The same tone as ROTLA, but not a 30s pulpy feel. Maybe even a bleak Western (think the Misfits starring Gable) sort of feel or Noir, but ultimately completing the circle--Indiana discovers God's covenant with man in the first film, and the last film brings us to God's first home for humanity. It'd be serious like ROTLA but with the depth of LC--Perhaps Indy loses his family but finds the birth of humanity and the beauty of God's plan, maybe ending with Indy joining his family there--to live on eternally in the realm of Eden.