// The funny thing with your elven city example is, most of the GMs I know would just go along with the suggestion, make a note of all the stuff you made up (name of elven contact, name of river, where it comes from, where it goes to, what the potion does) and rub their hands with glee thinking of all the ways they could use it to make your life “interesting”. Possibly in the “Oh God oh God we’re going to die” sense of the word. Not necessarily right now, but, oooh, let’s see, three sessions from now when you’ve forgotten about that elven contact, and she turns up and demands a really inconvenient and dangerous favour in return for that potion. Or when the potion’s long-term side-effects start to kick in…
//Everything a player invents belongs to the GM.
//There are lots of game systems out there that could be adapted. Do people have a favourite?