I am increasingly thinking that, in the possible scheme where multiple check marks are required to get a clue, check marks and leads go together. I first assert a well-worn design principle: rewards paid out by a game system should be commensurate with the opportunity cost paid to obtain those rewards. In this game, the reward is information, so the better the information you get, the more you should have to pay (or the harder it should be) to get it.
In the most recent version, we had leads but no check marks -- every card gives a clue. Since it was hard to know in advance how good of a clue a given card would provide, it was correspondingly hard to know which card to go for. The lead system therefore put a barrier in front of acquiring information, but the barrier was the same height regardless of the quality of the information.
In version 7, nine of the theme cards are out on the board at the start. All cards are equally good -- each gives a check mark which is worth one level of clue (although some have more categories than others). Since you had to visit multiple cards to get multiple check marks, there was an acceptable balance between opportunity cost (more trips to visit more cards) and rewards (more check marks).
The "problem" with this system is that it's a little boring, and that it doesn't differentiate between the cards. Yes, the Grail Diary has information about more subjects than Elsa Schneider, but if you're JUST interested in getting info about the Grail Room, they're equally good. It was not a bad abstraction by any means, but as this blog demonstrates, I'm always inclined to ask whether there's a better way.
Increasing the number of check marks required to get a clue gives room for differentiation of the cards. Now, the Diary can know a lot about the Temple Challenges, by giving 4 check marks, whereas Henry Jones Sr, with his somewhat shaky memory, only provides 2. But adopting the same approach as v7, where all the cards are laid out from the start, would result in the opportunity cost being essentially the same for all of the cards, despite their unequal rewards. Yes, board geography and the game clock play into this somewhat -- it may be better, for example, to visit Jones Sr in a nearby city, accepting the lower payout, than to burn the time/resources to travel across the board to see that Diary. But above and beyond this, it seems that the game has to "protect" the higher-valued information more aggressively. And leads are one way to do it.
A possible approach could be to have the lead system be city shape specific. Again, each theme card has an associated city shape, and you go to a city of that shape to get a lead to that theme card. We could have the lower-valued theme cards take leads that send the players to less-dangerous locations, whereas the leads to the higher-valued cards require you to travel to locales where you'll have to face a challenge. For example, a typical lead to a card like "Sallah", with minimal info, may send you to a major city, whereas a typical lead to a card like the Grail Tablet, with strong information may send you to an Enemy Stronghold.
In this way, the game can make the more difficult cards harder to acquire, but can do so in a completely organic way, as opposed to adding an additional cost or surcharge associated with accessing the better cards. And again, it continues to treat the "relics" as ordinary theme cards, instead of special theme cards with special rules. I think I like this; although I did like the mini-race element that the relics added, in practice I like the idea that all theme cards are functionally the same, and the game elements themselves differentiate between the various cards simply by the way their built, and not in a way that the players need to explicitly police. It keeps the bureaucratic overhead of playing the game minimal, and I think that's key to keeping the length and complexity down.