So my idea is mental inventory. It's like a regular inventory but instead of listing items, it lists things you've learned so far. Whenever you find out something significant (piece of obscure lore, geographic location of something important, personal information about certain characters etc.) you get an unique item symbolizing what you've just learned.
These can be used in several ways. First of all they can be sold to information brokers for money or other information. Information sold that way won't disappear from your inventory but it's value will drop down, after all the more people know about a secret the less valuable it is. The other use would be to affect dialogue choices. Some choices can only be accessed if you have a specific piece of information, like knowing about the mutant's infertility in Fallout one. Other times knowing a certain "value of secrets" will be enough. Having enough items tagged "political intrigue" which symbolize secrets you know about local nobility would be used to impress a local lord. Knowing enough "arcane secrets" would impress a wizard etc. Other times it could be used to make certain check easier. A guard will let you into a city if you bribe him or show him proper papers. But exchanging knowledge about far-off lands will lower his price.
The obvious question is: "why bother with all that crap instead of tracking everything in your journal like in a normal RPG". It makes playing a diplomat more fun, because instead of putting points into your speech skill you now need to actually think about what you're doing and play it a bit like an adventure game. You only run into a wall as a diplomat when you encounter a speech check that is too difficult for you at a given moment, the only way out of this situation is to reload and return to the conversation after you've put some more points in the relevant skill. If some additional information is needed you just need to finnish relevant subquests on your way. Now when you run into a wall you need to think about whom you're talking to and what information could this guy be interested in. Then you need to think where could you find this kind of information and hunt down relevant NPC. This would make playing a talker feel more like an actual game than a CYOA.