It would be cool if for adding a new field input type text you could provide 2 additional fields. First field would take a regular expression string, another just a text for an error when the value does not pass regular expression.
This way, those who know power of regular expressions could provide for fields a good checking of the inputted data (you could also provide a drop down with most common like numbers only, one numbers, emails, website, etc). However, just a custom regular expression would be very cool, most programmers could make up their own one.
I think this would make this plugin a much more powerful one since it would allow field data checking.