- A maintained, portable, stable, widely-available implementation.
- I'm iffy on whether Racket is widely-available enough for a Racket-based implementation to satisfy me.
- A module system.
- A package-distribution system.
- Better and more libraries, or a good, standard way to hook into racket's libraries.
- Depending on the project, a reasonably fast implementation.
On looking into the matter, it seems Racket is available on Windows and OS X, so that's basically taken care of. Hopefully its source compiles on non-x86{,_64} platforms as well, but I don't really develop for those.
I'd suggest just trying to compile it on whatever machine you might have in mind.
git clone http://git.racket-lang.org/plt.git
cd plt/src
./configure
make
make install # this builds executables and docs in place; takes a longish while