So if you looked up "Henry Jones" and found he had a relevance of "2", you might conclude that (a) it's worth visiting him to get his clue, and (b) you might want to look at the clues, or relevance, of the other theme cards that share some of the same categories.
We abandoned this system quite a while ago for reasons I can't entirely recall, but one of the big ones must have been that it's just visually much easier to look at and evaluate an array of 12 cards that each have 1-3 icons than it is to look at and evaluate 12 cards that each have 3 text fields. But what it did do, and what no version has done since, is communicate a sense of following a hunch by utilizing info about cards that may be "connected" to each other. It did this in an abstract way, but it still was a good first attempt in this direction.
Again, although complexity is the last thing the game needs at this point, I've been thinking again about something like a "relevance" system. Picture this: instead of each theme card having 1-3 clue category symbols (which authorizes the player to look at clues or get check marks in those categories), each card has 1-3 "areas of information", which represent "things this card might have knowledge about." Also imagine that each clue category gets a "hub" card associated with it, and when you want to ask a theme card about a clue category, you slide the hub card into the solution frame for that card's number, and it tells you what you get -- ie, the level of the clue you're authorized to look at, or perhaps nothing. And then from there, if you're authorized by the hub card, you get to look at the actual clue on the back of the solution card. (This two-step process is one of the main things I dislike about this approach)
Since the hub card "knows" which theme card is which, it also "knows" which categories are present on each card, and thus, cards that share a category could similarly provide access to clues. So if you determine that Marcus Brody authorizes you to look at a Level 1 Location clue, and he has knowledge about "Egyptology" and "Languages", you might look for another theme card with one of these same categories in the hopes that it will provide a clue.
But wait, there's more -- imagine that the hub card is physically hard-wired into the solution card in the same category. Then, the categories themselves can be connected to the solution. For instance, if Marcus' knowledge about Egyptology or Languages are "relevant" to the solution, maybe it's because the lost temple is (or was) in Egypt, or maybe it's because the "Coptic Manuscript" is important.
Such a scheme could do two neat things. First, it could enable a somewhat more comprehensive "solution", where different pieces of information can all talk to each other to assemble a more robust solution for each clue category (but yes, each clue category is still orthogonal to every other category). Second, each card could be less formulaic and more free-flowing. Sometimes, the association between the categories on a theme card and the element of the solution will be overt, sometimes it will be highly indirect, and the player's creativity and intuition may be required to connect the dots. The "brute force" approach of acquiring every clue will still be available, but a savvy player may be able to get to the solution more efficiently by relying on some guesswork, hunch-following, and utilizing the local rumors (to be discussed in another post).
Now there's still the potential for information overload, but perhaps it's not as bad if not every card has 3 categories, and/or not every card is out and available all at once. If the board starts empty, and theme cards are added as players chase down "lead cards", then you'll only be adding a few "categories" at a time, so it shouldn't feel as unmanageable.
Basically, it transfers the "problem" of finding out which theme cards are germane to the system from something that the game automates for the players, into one that the players must actively determine for themselves. It's probably too much complexity, of course.
The question really boils down to what the problem for the players ought to be with this aspect of the game. It seems there are four choices:
(a) You know what info the theme cards will provide and it's just a matter of finding where they are ("lead card" system only)
(b) You know where all the theme cards are, but have to figure out what info each will provide ("relevance" system only)
(c) You need to figure out BOTH where the theme cards are AND what information each will provide ("lead cards" and "relevance" added)
(d) You know where the theme cards are and what they provide, and must simply parse the information in the most efficient way (present system)
I'm inclined to consider systems like the lead cards and the relevance system because they feel to me like approaches that could add more cinematic scope to the experience of playing the game, that they'll make each playing feel unique, and that they could provide more variability to the different scenarios that the game could include.