I wouldn't exactly say it "trumps" all others ...
I disagree. "Holy Grail" is an established figure-of-speech representing a quest, usually an impossible-one. It was the obvious choice, far better than the McGuffin in Temple of Doom.
The Shikara Stones of DOOM were alright, as a "McGuffin," in relation to the almost devil-worshipping cult surrounding it. As I understand it, they really did exist around this time, though not as Hollywoodified as we see here - obviously.
Voodoo has nothing to do with any religion or cult in India, nor does Eyeball Soup or Monkey Brains or any of that made-up stuff we saw in DOOM. But had the script shown less ignorance of India and borrowed more heavily from Historical Fact, then, I suppose, the stones would've ended up being perceived as the only real "flaw" of the movie, as not being "interesting" or "worthy enough" for an Indy picture.
I don't know. But I, personally, have no problem with Lucas & Spielberg having introduced a "McGuffin" that nobody outside of India might've heard about. I don't think these movies MUST rely on Popular Legend.
Saying of which, aliens play no part in legend, whatsoever. Human History is replete with legends of magical, mystical creatures, like Dragons, let's say. But it was understood, even then, probably, that they were more a symbol for something, rather than a flying beast you had to be on the watch for. Regardless of any debate, the fact was, in the Indy Universe, aliens are "real." Aliens also, like The Holy Grail, are part of the Popular Culture and unfortunately, they decided to play it "safe" by using them.
Harrison Ford's so old now, it doesn't matter what you do with Indy, anymore. Back in the 8O's this franchise had great potential and could explore different concepts alot better and appeal to audiences on many levels. This franchise is just a Novelty Act as long as Ford's onboard. And I, personally, would not mind seeing them rebooting it. If they do, just please ... leave the Ark angle alone ... it will never be done that well, again!!!