CPython
CPython is the default, most widely used implementation of the Python programming language. It is mostly written in C, most of the modules are written in Python itself. In case you want to make the changes which will be used by all the Python programmers worldwide, this is the project to work for.
Contacting CPython developers
One can contact the core developers over IRC or using mailing lists. #python-dev (on freenode) is the channel for core developers, the core mentorship mailing list is the place to an email discussion. Remember that the developers are may not be in your timezone, so it might take time to get a reply from anyone.
Getting Started
The best place to get started is the devguide. You will find all the instructions required to build your own CPython. Before contacting the mentors, it is better you start solving few bugs first.
No ideas here yet!
We're still waiting to find out which mentors are available for Core Python/CPython projects. In the meantime, you can discuss ideas on the core mentorship mailing list.