I would like to, and I feel vaguely excited without having any particular ideas, but I don't know if I'll be able to finish anything. The last time I did was 4 years ago!

The adaptation is really close, so I find that most fanfiction can be read as either. The biggest changes are, I guess, the 1960s conversation and Aziraphale's fears about getting too close, which make it unlikelier in the show than in the book that they could get themselves together until after (or at the end of) canon. But there are still people playing around with that date who haven't even read the book, too.