To cater to my target audience, I will begin with the assumption that you're an anime fan. There are who-knows-how-many series out there, and some have more pages than others. Not to dissuade anyone, but I would be very cautious about creating a Sailormoon site. I have seen more of those than any other, which is difficult when the series I most often look for are Fushigi Yuugi and Rurouni Kenshin. But if you have a great idea, go ahead and do it because also some of the best sites I've seen have been Sailormoon ones. But of course there are many series that don't have many fan pages at all, or at least have some that are difficult to find. Unless you have lots of information or a very good idea, it might be advisable to go with a combination. Note, I say might be. This isn't always true but you can attract a greater audience with more stuff.
Once you choose a series or combination thereof, you need to filter it down a bit more. Is it an info page, a club, just a normal fan page? Also, will you focus on a a single character or group of characters? My favorite pages to see are those for specific characters but I don't know enough about any one character to make a page like that. But shrines, single-character clubs, or lots of info on a single character are very popular topics.