Sometimes information about a control itself needs to be referenced either by the control itself or other controls. The existing properties are not always sufficient. It would be highly valuable to be able to add custom properties to controls. This would allow us to store and reference them in formulas.
I've had to accomplish this previously by hijacking an existing property, but this is both unclear and problematic. It is unclear what the property is used for since I can't provide it a name. It is problematic in that sometimes I'll need to use those hijacked properties. Additionally, needing more than 1 can be a real challenge.
For instance, when I've needed an index value associated with a control I've hijacked the BorderThickness property. This worked because BorderStyle was set to None. But if I'd needed the border, I wouldn't have had anywhere to stuff my index.
Ideally, a control or group of controls could be selected and then there could be a plus button at the bottom of the properties panel. When adding a new property a user would supply a name and a basic type like text, number, boolean, enum, etc. These new properties would then be accessible in formulas and their values could be formulas like all other values.