The problem with TeX is that it gets most of its current power from the large library of packages written on top of it (or on top of other packages, e.g. LaTeX). People expect those to keep running on any new implementation of TeX. Which means that you need to be almost 100% compatible; you cannot really mess with the TeX language in any way. So the best you could hope for is a clean implementation (or whatever we perceive as clean these days) of a processor of an ugly (again by today's perception) typesetting language. It's not clear that that is worth the quite substantial effort.
You can of course aim for a new typesetting language and just take the typesetting algorithms from TeX, but then it's a completely different product, incompatible with existing TeX packages.
The packages are secondary; TeX stripped of all its accouterments already solves a really really hard problem, and is extremely fast and free to boot. The rate of bug discovery has slowed to about one a decade so there is really no need to mess with the internals at this point. It "just works". I think TeX will be with us for a very long time yet.
I think compilation to TeX is probably always going to be less successful than compilation from TeX—that is, taking the same code and (slightly) improving the processing. See the pdfTeX (http://www.tug.org/applications/pdftex) and LuaTeX (http://www.luatex.org) projects.
Alternately, you could rely exclusively on a TeX export utility and make a clean, less verbose language that compiles to TeX. Then existing packages could be either wrapped similarly or inserted as TeX blocks.
I think one reason most people wanting a next-gen-TeX project don't aim to compile to TeX is that some of the biggest pain points of TeX are baked into the core. So if you want to improve them, you need to change or replace at least some of the core layout algorithms, not just the front-end input language. For example a big wishlist item for many years has been some kind of improvement on TeX's quite frustrating figure placement, possibly with a more pluggable layout algorithm.
You can of course aim for a new typesetting language and just take the typesetting algorithms from TeX, but then it's a completely different product, incompatible with existing TeX packages.