Yeah, I wonder if setting up piece limits or even a material limit would help the blockading thing. Alternatively, have a "threat level" for the AI where it gets more aggressive with adding new strong pieces if you are just skipping through turns quickly.
I have a few ideas for challenge mode. Specifically a King of the Hill mode where you need to keep a king in the center of the board for X number of turns. It's just finding the time to make that happen at the moment.
Thanks for the thoughts and feedback on this!