@Stephen - Are you able to tell us if there are any other planner-related features on the roadmap? I know that I've seen several requests for "Create Bucket" action, "Task completed" trigger, and my personal suggestions - "Create Task" allows assignment to multiple people at once, and "Create Task" allows for the description to be input at the same time.
Triggers for Planner are now available, read more here: https://flow.microsoft.com/en-us/blog/planner-triggers-connector-improvements/