I also wonder if they could still base it off SVG, but just append to the spec for their own new format. It's just XML under the hood right? They already have all of the complex parsing set up for SVG XML documents.

If svgx files or whatever just had some non-svg node in the top level that contained all of the special inkscape-only info, you might even be able to keep it totally compatible with svg itself. (Just obviously, lacking the special inkscape features)

    <svg>
      <ink:inkscaperoot xmlns:ink="inkscapeschema">
        <effect href="#x">
          <border strokewidth="1"... />
          <border strokewidth="1"... />
          <border strokewidth="1"... />
        </effect>
      </ink:inkscaperoot>
    
      <g><rect id="x" />... regular svg...</g>
    </svg>