Currently, I can restrict a board so that only people assigned to items can see only those items and nothing else. This is great!
I'd also like the ability to do the same with sub-items. That way, I may have a sub-item for one specific project that they cannot see any other sub-items. This keeps the rest of the restricted project hidden, but allows that one sub-item to be completed by someone outside of the project.
Option to show or mask the main item would be a stretch goal.