I think you could tweak the card suits idea a little and get closer. Imagine the suits represent similar realms but experiences, skills, and opinions rather than exploitable secrets For example:
- ♦: Memorable purchases, moments of fame, dreams of success.
- ♥: Lost loves, stories of home and family, nationalism
- ♣: Tales of hardship, monster encounters, war stories
- ♠: Teaching of philosophies, religious doctrines, guild bylaws
How would this conversation be systematized? I don't think it needs to be. I think it would become very obvious to a DM if a player was trying to hit on three subjects in quick succession just to rack up experience. It would seem unnatural and might shut off the npc completely. A measure of trust and comfort would need to be achieved to broach more private topics and I think players and DMs would have a natural sense of that.
So, how much experience? I'm not sure. Maybe 100 xp for private ideas, 200-500 xp for secrets. I think it would need to scale with levels though, or you would find going up in level would require conversing with more and more people. Trollsmyth and Oddysey would best have a feel for the pace here: how many conversations feels like enough to go up in level. Perhaps becoming friendly and conversant with 10-20 people would be enough to move the character forward a level. So, multiply xp awards by level to keep the number of interactions consistent.
I'm having an intuitive tingle that letting players pick five things for themselves might be fruitful too. Maybe you get twice the experience if you find someone with a similar religious philosophy, or who has had similar traumatic war experiences. This would push players to explore npcs, searching for those like-minded souls.
I thought about giving experience for sharing information about npcs with other npcs, but I think that would just push towards gossiping. I think it will be useful enough to find out about person A from person B so that you could confront/or console person A about that subject.
Heck, this might even work for romance.
As far as the cards themselves, this could all work with them being completely discrete: a hand of 5 clubs is five separate experiences/memories of hardship. I think it could be interesting to make runs of the same suit or same card mean something, though. Showing similar ideas memories related to each other. The numerical value of a card could represent chronological sequence-- you discuss memories 3, and 4, before the person will reveal 5, or vice versa.
If you didn't do this and only wanted the bits of info to be discrete, what would the numerical values represent? They could be how much that person knows/has to say about it, or how willing/unwilling to share about that subject.
Anyway, I think it is possible, with simple, abstracted mechanics to push towards almost any type of play you might think of.