Seeing what else is in store for my favorite couple (adhering to canon while simultaneously exploring new ideas that don't tear through canon too badly) is always a revelation for me. That's like 90% of the fun. I have a wee idea of what I want to write--the high points, the low points--but it's the moments in between and how the characters reveal themselves that are the real joy.
I understand that these characters aren't mine. They have pre-established personalities, experiences, and traits I want to respect. I don't want to recreate Meyer's characters, I just want to flesh them out a bit.