For now, I'm just to create a list. Once my list gets a bit long, I'll think about what will go into each focus to narrow it down.
1. AI in Education (Intelligent Tutoring System)
2. Constructed Languages (Lojban) in AI
3. AI in Interactive Narration
4. Knowledge Acquisition Through Machine Perception
6. Using Lojban for Both Knowledge Acquisition and Representation (See Goertzel)
Updated (11:23): Removed 5; English, or any natural language, is a pain to work with and too large for PhD work. While I could break it up for both during and after the PhD, it's still the least appealing topic listed.
Looking over the remaining topics, I can't help but break them up into the general areas of study that I would have to learn about. All five require study of AI. Two and six require mastery of Lojban, a good knowledge of predicate calculus, and a respectable understanding of linguistics. One requires knowledge of education, psychology, learning theory, and things of that nature; I have some background here. Three requires knowledge of storytelling, something I'm a big fan of. Four requires detailed knowledge of the perceptions I use (probably vision, a very large topic). I could conceivably combine four and six (if I was insane) and use Lojban to classify the knowledge that I gain.
Even after choosing the general area and reading up on the topics, I still have to narrow it down to something much more specific. Preferable, I'd find smaller bits of research that can be done first to give me a head start and something to publish before the dissertation.
Lots to think about. As always.