For a project we needed a way to add optional walls into our drawing.

This is what we came up with:

How? Well, we used the phasing feature of Revit, where you can determine at what point in time a part of the building is constructed and/or demolished.

Ribbon > Modify > Phasing:

In the Project Phases tab I added a point in time "optional" before the actual construction time.

In the Graphic Override tab I changed the view-overrides of Existing to the dashed lines I wanted. (And for explanatory purposes I also set the Demolished lines to the same pattern)

I then went to the Instance Properties of the walls in question and set the Phase Created to "Optional".

You can then set the Phase Demolished either to "None" or to your construction time, with the following results:
The connection with the other walls is better when the wall is not demolished.
But on the other hand, rooms are cut off by this wall.

There may be other ways of doing all this, but for us, this provided the quickest and cleanest solution to the problem.