Games : Is there anything useful, besides executing and ransoming, that I can do with an imprisoned liege?

I've been playing as a Duke in the HRE, and I've managed to imprison my liege (the emperor). However, now that he's here, I really don't know what to do with him. My goal right now is to lower crown authority so I can annex some of my weaker neighbors, so if there was any way I could use this to help with that, it would be great.