We use Bazaar for source management.
You should use the TRUNK source tree (currently called "mysql-trunk") for all new developments.The public development branch can be downloaded with
shell> bzr branch lp:mysql-server
Before making big design decisions, please begin by posting a summary of what you want to do, why you want to do it, and how you plan to do it. This way we can easily provide you with feedback and also discuss it thoroughly. Perhaps another developer can assist you.