Eh, you'd probably use <rect> rather than a single <path>, because then you can trivially slice your sparkline and tag every tick with metadata. So for simple sparklines you don't really need a custom element. Page elements already follow a box model, just CSS them boxes =)

But for an annotated data set that renders as a sparkline, a custom element with an SVG shadow DOM probably makes sense. Definitely more work than the "this'll do for most folks" solution though =D