Sorry I was trying to follow your meaning, because the example of absent rich text you gave was underlines. There is already basic text formatting support such as strong, emphasis, headings and links.

You could style any, all or none of those to have underlining. Whatever chosen rendering, they all have meaning independent from the applied style. What you are asking for is to have something that is purely a display style without any structural value. This is not coming to markdown any time soon. Hardly anyone uses underline as its own thing in html anymore, for good reasons.

Maybe this article will help to further explain.

Native SVG handling would be completely out of scope. The point of markdown is that it is supposed to be understandable in its plain text format. SVG is incompatible with that. The closest thing would be like mermaid charts but I think it's quite a stretch even then.

I think you should just use HTML, it has a much larger array of tools that would suit your needs. Markdown is purposely constrained because it enables much more portability.