Allow attributes in DataTypes
DataTypes should support attributes to create structured types.
For example: http://www.uml-diagrams.org/data-type.html
I think I understand your comment. Apologies if I just missed the whole point...
As a matter of best practice, it isn't a good idea to create attributes with structure. The database books will all say that an attribute should hold a single indivisible chunk of data. For example, if you let an attribute CustomerName hold a string for the customer's first, middle, and last names, you can never query the database for all the Customer instances with first name "Jane." My suggestion is to let a Class do the work of aggregating data things together.
Reece Hart commented
To be clear, I mean that when one creates a Data Type classifier, the frame does not have the hover options to create attributes the way Classes do. In fact, I can't figure out how to add attributes to Data Types, even though I think you must have intended to support them.
It might also be handy to have a way to convert Class → Type and vice versa.