A note: I've gotten a few suggestions lately about starting to make character specific books instead of fandom specific books here, as most of my readers are here for only a few characters and will skip the rest, and me showing up in notifs with "wrong" characters is annoying + readers overall would prefer reading books that only have fics for characters they like.
Yes, I agree on how it would be more convenient for readers. But for me, it would be a nightmare, even more than now. If you take a look at my profile, I have 45 books, which means there's 45 fandoms I've written for so far. Each book has fics for different characters, at the moment I have written stuff for 219 characters, which would make 219 books.
I know that for readers, that wouldn't be a problem. But updating those books would be such a chore, I already have to wade through the Wattpad book edit page thing ("my stories") whenever I publish a new fic for a fandom that's not among my most popular ones, and the page is really buggy. The books take a while to load because there's so many and I have to manually click on "show more" after each load. It also often jams and I have to start it again. So obviously with 219 books, it would take even more time, would be more frustrating = I would stop updating again.
So, I hope you understand why it's impossible to do at the moment at least. Again, if you wish to only get notified when I publish a new fic for your favourite characters/want to browse through fics where only your favourite characters are the main focus, my AO3/Archive of Our Own is the only place to go. (Also a reminder that the website is completely free and has 0 ads)