...honestly, I'm so sick of seeing this mentality. Ford's age is (technically) irrelevant to what can (or can't) be featured in the fifth film. In this modern world of CGI, they can technically make Ford look like he's doing anything. Body double with his face on it/full CGI character/puppet him with strings and just CG erase the strings/whatever.
Now should they? Oh God, no. But people claiming "he's too old so Indy can only sit around in the movie" is such a lazy argument.
And also, as I've been elaborating on before... just because it's an Indiana Jones film doesn't mean that they can't construct a satisfying movie that works within the limitations set by Ford's age. Let Indy have a limp, if Ford really does have one. Who cares?
If someone is so particular in the sacredness of their memories that seeing Indy as a old man ruins things for them... just don't see the movie. I love Star Wars, but I have no interest in a show about some random Mandalorian.... so I won't see it. Its existence does not harm me.
I'm all-for discussing an idea's merits, but when absolutely no idea seems good enough for someone (not talking about anyone in particular here, just in general), then why bother joining in the discussion?
Now back to the tone--I would hope that without George involved, they'd be willing to branch out again in terms of style/content. I think it should still feel like an Indiana Jones film. I wouldn't want it to suddenly feel like Inception or anything. But I think you can bend and stretch the conventions of the character/style to get a different feel. Clearly, Ford won't be in another one after 5 (assuming it actually happens), so I think it's necessary to give the film a little bit of a somber feeling, lend it an edge of finality.
The tone of ROTLA is so necessary on this one not only because it would bookend the series in terms of tonality, but also because it's the right canvas to mix-in some melancholy. ROTLA was mostly moody with serious characters and a palpable sense of threat. TOD had vast portions that retained this, but by then we added comedy sidekicks. This is different than ROTLA (and KOTCS too, technically), because comedy with sidekicks is different than "comedy sidekicks." Short Round and Willie generally were used expressly for comedy. Willie's entire character was predicated on it. LC had turned Sallah and Marcus into comedy sidekicks, and Henry Sr. was both A source of humor and used to make Indy sometimes bumbling. The tone of the second and third films are drastically changed by these facts.
KOTCS goes back to having sidekicks that have quirks, but are generally played straight. Where KOTCS veers off from matching the tone from ROTLA is in the situations--because of CG they were usually much broader and thereby more outlandish--whether these were good or bad is up for debate, but it still prevents the original tone from shining through. Don't forget that ROTLA was nominated for Best Picture. Despite the comic book nature of the ideas on screen, it played ominous and moody with a dash of intrigue and suspense without losing a sense of fun.
Lightning in a bottle, boy yes--but I don't get the feeling that they ever actually tried to recapture that exact spirit. For Indy 5 to fly, I think it's time they zeroed in on that target one last time.