It would help a lot to have masking in the Text Entry widget, that will allow 
the user to define a certain format for the input and enforce it while typing. 
This could be useful for time and date fields, IP address fields and in many 
places this option fills a specific demand (where I work we need this option 
in about 4 or 5 very commonly used fields). Since the development I do is 
using GTK#, and since extending a widget to support such an option is not an 
easy task (I tried), and since it is a very demanded option (at least from my 
research), this would be a very much nice-to-have feature.