Whilst not a priority it would be very useful to be able to set validation on the custom customer fields as created in Settings, Customer fields.
For example field type create as Date - this would only permit valid dates to be entered , optionally permit future, past or any date.
Field Type create as Numeric - input must be numeric optionally set to 2 DP
A more complex but really wonderful option would be to allow the creation of drop boxes, Take a look at how Zendesk or Freshdesk do this. Basically both software systems allow drag and drop creation of drop boxes and dependant lists.
The introduction of validation will make the data held in custom fields much more reliable, and eliminate typo's if the field is being used to categorise or date something
Now if we could get that those custom fields into invoices and email templates as substitution well........
That would be really brilliant.