After some quick googlin', I discovered that many of the terrible Disney sequels were actually passed off to other writers. There's a set name of writers that do the good stuff, and a set name of writers that generally come up with the bad stuff. I guess Disney was really stupid about that.
Luckily, Frozen 2 is going to be written by the original Frozen's writer. She was actually the only writer, whereas many older Disney films have at least three writers.
So, I'd like to think that will make this not flunk like Disney's other sequels. We'll see.