I want to think that ConTeXt is that successor. While has a slightly different markup from LaTeX but is still familiar enough, I find it typesetting/markup to be both consistent and flexible.
It has the added bonus of being a monolithic tool —it packs everything one needs to typeset a document— but it is not as adopted as LaTeX, so there is not a lot of material on it.
It has the added bonus of being a monolithic tool —it packs everything one needs to typeset a document— but it is not as adopted as LaTeX, so there is not a lot of material on it.