One possible advantage of the "relevance" system I proposed (whereby each theme card has three "subjects that he/she/it knows about", rather than 1-3 "icons corresponding to clue categories that the card will definitely give you a clue about") is that it could enhance the flexibility of a given deck; specifically, it could enable creation of multiple adventures using the same deck of theme cards.
Although it's quite premature, I was thinking a bit about what would be included in a published version of the game, mostly to make sure the component requirements aren't completely off the deep end. I think they're certainly IN the deep end...
12 Theme Cards
24 Lead cards
24 Location clue/solution cards ("category A")
12 "Adventure 1, category B" clue/solution cards
12 "Adventure 1, category C" clue/solution cards
12 "Adventure 2, category B" clue/solution cards
12 "Adventure 2, category C" clue/solution cards
It's a total of 108 cards. This suggests a format for possible expansions. Each could include 108 cards: a new deck of theme cards, and two adventures that you can play with those theme cards, both of which share the same lead cards and location cards. (Of course, the adventure cards, encounter cards, and identity cards add another double-deck of 108 cards to the base game).
So for each theme deck, the game provides you with 2 separate adventures you can play, and because of the nature of the solution cards you should have nearly unlimited replayability from each adventure. (A different version of an expansion could use the same theme deck and just add 2 new adventures' worth of clue/solution cards, 48 total cards instead of 108). Granted, for a game that emulated the IJ films, the ability to play multiple adventures with the same theme cards may be somewhat beside the point -- presumably you want to choose between "The Last Crusade" and "The Temple of Doom", not "The Temple of Doom" and "Some Other Adventure That Involves All Of The Same Characters As The Temple Of Doom" -- but in the somewhat likely scenario where the game gets re-themed to a more generic setting, having the same theme cards participate in several adventures won't seem as problematic and may even seem like a feature.
Incidentally, the player who will say "I want an expansion that offers MORE theme cards in a given adventure (eg 18 instead of 12), to make the puzzle even MORE complicated" -- well, that person probably doesn't actually exist, but if he did, sadly this can't be accommodated in the way he wants, because we only (plan to) have 12 solution slots, so 12 is always the magic number. BUT, we could placate this person by allowing TWO adventures to be played concurrently, each with its own set of theme cards and solution cards (but presumably sharing the same lead cards and encounter cards).
The best way to do this, I think, would be to add another full-size, 12-city board, which features The Americas (the main board features Europe, Africa, and Asia), and to set up the boards on two separate tables. The players would play normally at one table or the other, but would also be allowed to switch to the other table, if their clues and information encourage them to do so. It would a be huge, time-consuming, and extremely difficult thing to play, but would require almost no additional effort on the design side other than coming up with an additional map (which we already have in the works). Realistically, adding an Americas board and the associated theme, lead, and solution cards, probably requires adding a separate stand-alone game (with identical rules), and you can combine the two games for a mega-adventure.
Obviously, that's way downstream from where things stand at the moment! But I'm at least thinking even at this early time about how to make the game compatible with such an approach.