It'd be great to be able to assign a drop down list of predefined arguments to set for custom parameters on templates and windows. SO when a template is selected in the designer the user can only select from the list of arguments for that parameter, or perhaps have the list but also be able to enter another parameter as you already can now. Similar to how a user can select colors from the color picker for color parameters.
Example being I have a custom parameter on a template which is a double. And this parameter is binding to the angle parameter of a graphic inside the template, then rather having the designer enter anything from 0 - 360, they can select from a drop down "up, left, down, right", which would correlate to "0, 270, 180, 90".
A way to help conform designs to internal standards.