The section filter dropdown specs need to deal with at least one tricky problem. Within the Section Info tool, when you create one or more sections, you must assign the sections a category. That category name, e.g., discussion, plus a number is the default name for the sections. You do, however, have the option of changing the name of each of the individual sections created within a category. This flexibility leaves the ordering of the sections within the single gradebook as an open question.
Below are two images with proposed spec to get the discussion going. These have not really been thought through.
Single Category Dropdown
Straight alphanumeric list, no category select option.
Mulitple Category Dropdown
Sections remain grouped under their categories All Discussions, All Labs. The categories are a first order sort. Sections within the categories are sorted alphanumerically.