So what might have been Elan's super secret plan?

[Kardwill]

Well, as prequels tend to be made after the original film, and take place chronologically before the original film, and given that you readily admit that these two facts apply to TGTBATU, I'm more than a bit confused as to how you don't already acknowledge that it's a prequel.

It doesn't really feel like one, though. Like Indy 1/2, the Dollar movies feel like different movies that just happen to have the same iconic character.

In fact, are they even in the same continuity? If I remember right, Lee Van Cleef plays 2 different characters in "good, bad, ugly" and in "for a few dollars more"

But overall, it's difficult to pull a continuity on iconic characters like The Man With No Name, Indy or Bond, since, by definition, they stay true to their "icon" and don't really change.

Fun idea: Tarquin's defeat (but not the plan leading up to it) is covered in a bonus strip. He goes to Hell and finds a copy of the book, which shows that not only is his demise not included in the main story, it takes up a single page.

I'm willing to bet that if we see Elan defeating Tarquin, it will take exactly one single box ^^ (possibly with Tarquin tied up and outraged)