What would be really nice is if there's code detected on the ticket level in the Source tab block a transition if a Pull Request has not been submitted. Similar to how you can block a transition if a code review in Crucible hasn't been completed yet.
This would prevent developers from moving their tickets when they have not submitted a pull request. You could also do one better and have the transition fire off an automatic pull request for the branch that is identified in the Source tab (if that still will exist with Stash).