AI planning, with a focus on plan quality optimisation; optimisation and AI for decision support.
Some potential student projects are listed here. These are mainly intended for undergraduate, honours and masters research projects, but some have the potential to be PhD topics.
Publications and resources
Nir, Dan, Christian and I have written a book on PDDL.
There is also a much shorter tutorial on Writing Planning Domains and Problems in PDDL that I wrote a long time ago.