I'd love to be able to see estimates in the calendar view. If a task has a due date then the estimate would extend back in time so you can estimate when you need to start the task. If the task has a start date the estimate would extend forward in time towards a potential due time.
If a task has both start and due dates, it would be good to have estimated time indicated in the calendar along side so you can predict where the gaps and shortfalls will be.
In the Unscheduled Tasks list to the side it would be great to have an indication of estimated time (and be able to sort by them) so I can easily see when I'm being too ambitious planning my day or week, and also so when I have gaps in the week I can quickly find a task to slot in there