In my view, rule-based systems has great potential of applied research. The Rule-Based System I developed - as part of my PhD - at a USA based healthcare IT company is still being actively used at the company (even I left the company in 2012).
I recently published a paper in 'Expert Systems' journal (IF 1.18) on evaluation of Rule Based Systems.
Within Rule Based Systems, research can be done on Inference Engine Design,
Supplying knowledge to RBS by using data mining techniques,