I am still a fairly novice DT user, so I am sorry if the application already has these functions and I’m not aware of it. But I’d like to suggest you add two column types:
A date column that can be modified by the user. This would make DT more useful as a journal/diary application.
A rating column, where one can select from one to five stars (or whatever symbol). This would allow someone to keep reviews of books or movies.
Of course, totally user customizable columns would be great (if someone says DT already has this function I will be both embarassed and happy at the same time).