We usually limit to a single assignee for each task. But that assignee changes as the task is handed around throughout it's life. We would like to have the ability for everyone involved to provide an estimate without having to create subtasks for each person.

This would work in single or multiple assignee mode regardless of who is/has been an assignee. Estimates could be added by any team member for any other team member ahead of time.

Time tracked on the task would then be put under the current user's estimate if one exists. The total estimate displayed would then be the sum of the individual estimates on the task (and subtasks if present).