This would be a need for our team as well, we currently have Scrum teams who utilize the Agile board. But we also have some team members who work on Kanban teams and they are not time boxed to Sprints like the Scrum teams.
The feature we would love to have is when using a Kanban board the ability to remove a task from the Story Board once the task is complete. We experimented doing year long Sprints but the board quickly became flooded with completed tasks that we couldn't remove. We currently are doing month long Sprints and move the "To Do" and "Doing" tasks to the next iteration once a new month starts.
Ideally, we would love the ability to have one continuous board that has the ability to clear completed tasks but still provide the ability for reporting capabilities :)