Well, how challenging do you want it to be? There are lots of some (Mickey, Tink, Donald) and less of others (Walt, movies, Country Bears).
And keep in mind...
no matter what you decide on, it won't matter, because when you start trading and looking at all the pins inevitably you'll find something else you like!
My youngest, Brooke (PoohTrader), started collecting Pooh and Friends because she already liked them (her room is decorated in them, has the beanies, etc). Imagine my surprise when, after a few months of collecting, she announced she wanted to collect Monorail pins! I never would've guessed that one.