-Writing a book with multiple protagonists. This doesn't entirely count--my second novel had six viewpoint characters. But it was not a very good novel, and I was 15 when I wrote it. And every other book I've written has only had one viewpoint character (a few of the short stories have had two). So I really should try it again some time.
-Writing an omniscient narrator. I always write limited, even in short stories. Always, always, always. Being in more than one character's head in the same scene makes me nervous. Which is exactly why I should do it.
-Writing a trilogy/series/something of that nature. I would really like to do this. I would even like my next project to do this. Unfortunately I have this habit of wanting to deal with all the important conflicts in my protag's life at the same time, and then there's nothing left for later books. Granted, Signi was intended to be a quartet, but I only wrote the first book, and I suspect if I ever go back to it, to make it better I will have to stick the stuff I wanted in the last book into the first and make it a stand-alone.
-Branching out in genre. As YA novels go. I've written pretty much every branch of spec fic in my short stories, and some non-spec stuff, too, but my YA novels (which would be the last four novels I've written) are all urban fantasy/real world with a little spec thrown in. I want to write a futuristic science fiction YA. I have tried (and so far failed) to write a high fantasy YA.