Being able to restrict a date to either the Current date or a combination of Current and Past dates, Past dates only, Current and Future dates only, Future dates only, etc. would be a very handy ability to include in Collector.
In my scenario (similar to what is discussed here: Collector for ArcGIS: Restricting a date field to current date), we want people to update the Start and Finish dates of forest harvesting operations as regularly as possible, but there may be delays due to the nature of our work meaning backdating is required. Projecting forwards though should not happen, so it would be good if we could disable that (Current or past date only). It would also be good if a 2nd date field could be restricted to not be back dated beyond a date set in a previous date field (i.e. End date of a harvesting operation could not be set as before the Start date).