Brilliant idea! I wrote one about five years ago as a birthday present for a friend, starring her as the "you" and beginning in the real world but gradually getting more and more absurd as about 20+ fandoms and tv/movie casts meshed with the various plots. I think I found it a lot less difficult to write than I expected - basically I just started from page one, and gave 2 - 3 choices every page or so, occasionally linking choices to previously written stories and killing "you" off when it felt like a plot was stalling. It ended up being about 15,000 words, but the whole process was so absurd and unpredictably it didn't feel like a chore to write.

Of course, a canon-compliant, single-fandom story would be a lot different I imagine. Nevertheless, it sounds like SUCH a fun idea. My one piece of advice would be DON'T think ahead - don't try and plan where the stories are going. You'll find they grow in wonderfully unexpected ways and entwine together on their own.