Version 3.0.3
None so far.
The matplotlib codebase predates PEP8, and therefore is less than consistent style-wise in some areas. Bringing the codebase into compliance with PEP8 would go a long way to improving its legibility.
Some files use four space indentation, some use three. Some use different levels in the same file.
For the most part, class/function/variable naming follows PEP8, but it wouldn't hurt to fix where necessary.
The implementation should be fairly mechanical: running the pep8 tool over the code and fixing where appropriate.
This should be merged in after the 2.0 release, since the changes will likely make merging any pending pull requests more difficult.
Additionally, and optionally, PEP8 compliance could be tracked by an automated build system.
Public names of classes and functions that require change (there shouldn't be many of these) should first be deprecated and then removed in the next release cycle.
PEP8 is a popular standard for Python code style, blessed by the Python core developers, making any alternatives less desirable.