Just so I'm clear - are you saying that because you want the franchise to keep going, or because you are resigned that it will keep going, whether you'd want it to or not?
I guess my view is - Indiana Jones is a character, not a "universe". Just like any character (or real person), when his story ends, it ends, Taking away the title character, there's no real "universe" or franchise-specific mythology left to explore (it's just our world, from many decades past, with the IJ character being the only unique aspect).
My view is that Harrison Ford embodied Indiana Jones - when he no longer is able to play the part, there's really no external reason to continue, no "story" left to tell that can only be told with this character in this universe.. If you (meaning the studio or the audience) want a swashbuckling adventurer chasing historical relics, feel free to come up with a new, different one (just as many novelists and filmmakers did both before and after IJ). There's no compelling reason to continue this particular franchise at that point, or to continue to use the "IJ Franchise Name" to shove a new character in.
M