Seriously, though, if there is another INDY film one thing we already know is that it'll play off of some kind of classic B-movie motif.
The original trilogy films were all obviously riffs on 30s Adventure Serials. KOTCS was the 50's era Sci-Fi B-Flick. I suppose they could go there again with the next one, but I truly doubt it. So what does that leave us? What were the B-Movies of the 60s (ones that Lucas/Spielberg would care to homage)? I don't see Blaxpoitation or a Bikini Beach Musical in Indy's future. What else is there from that era? The Hammer films are an obvious choice, but perhaps there are others that I can't key in on for whatever reason.