You can still create an anime fan club, and do just that if you have a small group of friends who like to discuss those very multiple series in one thread.
But separate threads for separate series keep discussions open to wider public, and attract more detailed discussions when needed. It's more organized too and anyone at any point can check them out for relevant information or discussions regarding it.
Thus, the series which have too many discussions and bigger following have their own sections and two more boards for the rest:
Seasonal anime section here:
https://animebase.me/forums/seasonal-anime.641/
& General Manga Anime and Comic section, MACBase here:
https://animebase.me/forums/manga-anime-comics-discussion.8/
If there is a series which is popular enough among a smaller group but not enough to bother with weekly threads, you can make a general discussion thread or/and let me know. I can stick it in the section, or make a sticky for it too if need be.