(1) I think it would be nice to have an automatic model generator. For example, you build a form panel with all your fields and field IDs, then you add a Model and right click and choose "Generate..." and Designer would scan your project for objects that have TextFields/ComboBoxes/etc and create the model with the field names as the field IDs. By extension, it would be nice to be able to have basic database access that could generate models from database tables.
(2) In the Project Inspector, it would be nice to be able to right click on the colored icons and choose "Add" where appropriate. For exampe, "Add Model" would be great, instead of filtering the toolbox for model, then dragging the model across the canvas and dropping it on the orange Models icon.
(3) Since Designer enforces a specific pattern of building MVC apps, I think it would be good to auto-generate as much skeleton code as possible. If you add a model, then you will most likely need a store. There should be a setting to allow a store to be generated for you and link to the model automatically.