I added the slope lines and constrained their end points to the planes. That was fine as long as the floor drain was placed separately, but I realized that automatically locating it at the intersection of the slope lines was a challenge.
After some deliberation, it occurred to me that the drain location could be established parametrically, and if the slope lines could be constrained to the corners of the object, I might also be able to lock them to the drain annotation object. This was accomplished by placing two grouped reference lines and constraining the slope line end-points to their intersection.
New 'Offset' instance parameters were established to control the location. The drain annotation was aligned and locked to the reference lines.
I assigned the slope lines to a new subcategory 'Slab Depression Slope Lines' to allow view-specific visibility control.
When the object is highlighted in the model triangular grips appear for all parameters. I have also revised the original rectangular slab depression, incorporating these improvements.
Revit is expansive.
For all three families download GEN_Slab-Depression.zip from the Files and Families tab above .