I have found a few instances where it would be nice if a single select or multiple select custom field and could have new selections added while filling out a task.
For example; If we have a project list and we are tracking the vendors we work with and one of my coworkers start to work with a new Vendor, then while adding a new task they could type in their name in that field on the task, and it could be added as a selection for future tasks
Example #2; if we have a list of sites we support and need to add a new one, then we could do it while filling out the task details and now it allows us to select that site for future tasks.
I think this would be helpful for items where we might use the same entry for multiple tasks, like a site name. Right now we manage our Sites field as a text entry field, which could be prone to spelling issues or abbreviations for the same site. The downside I could see is that field list getting pretty long, but being able to filter the options as you type is pretty nice.

