To draw lines instead of a geometry shader you can use instancing, since you know how many vertices you need to represent a line segment's bounding box. Have one vertex buffer that just contains N vertices (the actual attribute data doesn't matter, but you can shove UVs or index values in there) and bind it alongside a buffer containing your actual line information (start, end, color, etc). The driver+GPU will replicate the 'line vertex buffer' vertices for every instance in the 'line instance buffer' that you bound.

This works for most other regular shapes too, like a relatively tight bounding box for circles if you're drawing a bunch of them.