Aaaaah, okay. So, first off, as soon as the whole group of Nightingales was introduced, I wanted the adventures of Mercer, Gallus, and Karliah being awesome thieves around Skyrim.
So, I figure that, game-wise, Brynjolf is at oldest in his mid-forties, so 25 years ago he was, at oldest, 20-ish. But I sort of headcanon that he was this maybe 16 year old pickpocket who Mercer finds on the streets and realizes is pretty good at stealing things. And then he tags along on jobs (not on Nightingale jobs, but with either Gallus or Mercer or Karliah) and learns how to steal things and sneak everywhere.
I figure Mercer and Karliah would bicker all the time, and Gallus was fairly good natured about it and missed most of the worse barbs that were thrown around, and Mercer wasn’t always stealing money from the guild but started up at one point and realized that it was something he could get away with.
I need to come up with some ideas for Nightingale adventures…