> >> I make use of a feature that in a project that consists of
> >> several files allows me to compile each LaTeX file individually,
> >> thus saving some compilation time while writing the
> >> manuscript. I think such a system might be useful for inclusion
> >> in LaTeX3.
>
> It would be a nice feature, I think, and I have written some macros
> to do that. Currently I'm working on a new class which will include
> them.

how different from the conditional stuff and the combined documents
stuff that i mentioned?

i.e., what sort of specification are you working to?