More date qualifiers
Although most people would have no idea how to use them I'd like to see more complex date qualifiers and options.
For instance "Calculated" for birth dates that are based on age at death or some other point.
Dual dating would be nice but it confuses many people. (e.g. 20 January 1708/9)
Quarter dates are useful for people working with British Civil Registrations Indexes (e.g March Quarter 1856)
The most useful would be "By" rather than just before and after. If someone's is buried on a date, it's not given that he died before that date but he certainly died "by" that date.