Patterns for integrating rules and imperative (or other legacy) code.
Most interesting pattern(s) for replacing legacy logic: How to refactor, replacing procedures with rules (i.e., how to delegate decisions or other computations to rules).
...a few correlated "seed" thoughts...
"Black Box" rules engine interfacing: Replacing procedural methods with rule-based methods.
Rules as plug-ins?
Maybe something along the lines of "closures" for rules?
Encapsulating rule sets in various containers? Invocation by method calling? Invocation by messaging?
Metrics for rules integration? How to maximize cohesion and minimize coupling.
Quality Assurance... Evaluating rule sets for consistency and completeness?