The fifth Indiana Jones movie will hit theaters on June 19, 2019. With another decade having passed since Kingdom of the Crystal Skull, it's unclear how they intend to proceed, but I think they could simply look to the current action films for inspiration.
When Indiana Jones 5 comes out, it will have been 11 years since the fourth installment. Considering that Kingdom of the Crystal Skull chose to reflect the actual passing of time since Indiana Jones and the Last Crusade, it's safe to assume the fifth film will do the same, which means we can expect to see Indiana Jones dealing with life in the late 1960's. 1968 more specifically. Of course, Steven Spielberg could look to what was happening in that era and build his story around that, but it might be a smarter choice to play off of the emerging trend of modern action/adventure films:
Embrace the R Rating. It worked first for DEADPOOL, and now with the tremendous box office of the gritty X-Men spinoff LOGAN, it's clear that audiences want their adventures to be more adult.
Assuming they haven't got their script finalized yet, I'd like to pitch my own vision of the fifth Indiana Jones movie: