I do a lot of work that involves visualizing BIM models of planned highways using web scenes in ArcGIS Online. The way this has been done so far is by using 2D features draped over elevation layers exported from ArcGIS Pro.
One challenge that frequently comes up is that steep/vertical sections come off as jagged, in particular in medium scale levels. Multipatch scene layers render perfectly, but they clip through the basemap.
It would be great if it was possible to mask sections of the basemap using a 2D polygon. That way you would have more flexibility in creating scenes that involve complex surface geometries.