I know it is extremely late in your schedule for SVG 2 to make a suggestion, but I only recently came across a severe limitation of SVG, which I would enjoy being addressed in the new standard.
As of yet, paths and polylines are treated not as lines, but as surfaces enclosed by the said line (and possibly an implicit last stroke to close the shape). Which means that a group of paths/polylines, together enclosing a zone, do NOT actually enclose the whole surface when being treated by SVG. See joined file for an example of what I mean.
My suggestion would be to offer an additional tag, maybe something like <shape></shape>, that would contain polylines and paths, each of those being either an outer or inner boundary of the whole shape (using an attribute, or <inner> / <outer> subtags). This whole shape could in turn be treated as any other shape, being filled, serving as clip-path, and so on.
Each colored surface is a path filled with that color, although the boundaries between departments are common, and the colored regions (not visible here, but present in the SVG) are different paths, although regions are nothing more than groups of departments, and the coastline is again another path, which in turn is not the same path as the the international boundaries.
And there is no simpler way to do it as of yet! Not only would such a feature make SVG maps simpler, but also lighter: the whole code would be around half as long if only we could share boundaries between different shapes.