What makes a fic-able fandom?

In other words, what kind of fandoms are easiest to write fic for? What kind of fandom do you have the most fic ideas for?

Personally, I find fandoms that have a huge cast of diverse characters or maybe a lot of missed opportunities easy to write for. On the other hand, fandoms that have absolutely perfect source material or a really small cast of characters are hard to write for, to me.

I'd love to know your thoughts!!

Sorry if this had been posted before!