Retrospective topic: What do we measure?
You may know the phrase "You get what you measure."
If this is true, what do you expect to get? On time, on budget? What about impact? What about solving specific problems? What about learning?
What are you measuring? What should you stop measuring? What needs to be measured? How might you measure progress towards your goals better?
This is probably a brainstorming session, with follow on work coming out of the retrospective. It's a great opportunity to think about an important system condition that affects the way you all think and operate.