Overrides by category, filter and element all allow objects to be made transparent. However, there is no method to make an object opaque. There are a number of ways it could be implemented, what seems the simplest (in a GUI sense) would be to make the transparency slider go from -100 to 100. An override would default to 0, and the items could be made more or less transparent.
It would further help if subcategories could have their transparency controlled independently of a their parent categories.
Use cases:
- Need to identify the building mass in a broader site plan view. All building elements are overridden to solid black, but the skylights, which have a transparent glass material, can't be overridden. The only option is to place a filled region over the skylights in each view, since the material needs to be transparent in other views and there is no way to show the roof as "uncut"
- In a 3d view the glass is needed to be less transparent in order not to extenuate that the buildings surroundings aren't modeled
- Overriding something someone in a linked model has made transparent