I'll create a separate answer for each ToDo item. That way any potential discussion can stay localized. Whenever a task is completed, the corresponding “answer” can simply be deleted. Please add any additional things that need to be done/decided.
Any private macros need to be scoped (with which prefix)? Any public macros should be named in such a way that they are unlikely to conflict with another package. The same holds for pgfkeys handlers (I'd suggest putting them into a separate folder, instead of simply into
Make sure that everything works as expected.
Add a README file, that is displayed in the CTAN listing.
Make sure everything is documented. Should the documentation of the individual subpackages/libraries be put together into a single file? If so, we also need an introduction.
Decide on a versioning scheme for future releases.
Test PGF version
The package should test the pgf version and emit a warning if it isn't v2.10.