If Indy 5 is set in the 60s, should it deal with whatever B sort of movie was around in the '60s? Would Westerns be considered the "B Movie" of the 60s, analogous to what Alien movies were in the '50s?
Blaxploitation films are the B Movies of the 70s, and I mean if they really wanted to pursue a "Mutt Williams" series, that could era be believable, since Mutt would only be in his mid-late 30s (he was born in 1938 so a film set in 1975 say would have him at 38). Really they could make one last real Indy film, set like in 1959 perhaps--That would be Indiana Jone's last ride so to speak.
And then have the "Adventures of Mutt Williams" or whatever pick up say around 1961 and continue maybe to the mid 70s. A separate series not named Indiana Jones, but Harrison could show up in a cameo or something as an older Indy to give advice to "Junior."
Or, perhaps the first Mutt film could open with an Indy adventure as the prologue--separate from Mutt's adventure--where Indy finally meets his demise in 1961 while battling the bad guys for a treasure, and Mutt's quest is both to capture the treasure (or another, more important treasure) and also avenge his father. It would be something similar to "Utah 1912" where it's really a "Fedora" adventure interrupted by a young Indiana Jones--The first Mutt film would start off with a prologue similar to Raiders--separate, but with the main villain consistant throughout the film. It could either be like that, or something more like the "Mask of Zorro"--Either way Indy would meet his demise and Mutt would finally earn the "hat" (either literally or metaphorically) by showing himself to be the rightful heir to Indiana Jones. If they follow the "Mask of Zorro" route Indy would be a major force in the film but die in the middle or end, and perhaps it could include two separate but somehow connected quests, maybe Indy trying to recapture the Ark and something involving the Garden of Eden.