I’m in love with the nostalgic feeling that’s invoked by stumbling on old campaign notes. I’ve often attempted to build that into my games deliberately.
One of my current projects is a game about bards writing an epic about a hero. By the end of the game you’ll have collectively written a poem or song that you can keep forever.
A backburnered project involves collaboratively building a wiki that describes various aspects of a fiction, using the interlinking component of the wiki platform to connect everything. In the end, you have a library of information related to your fiction.
Collaborative map drawing games do the same thing.
What other games have a built-in keepsake?