1. Write up a proposal somewhere (Eg: in your userspace on wiki then
share the link around or post it to the mailing list)
2. Apply for commit access (link to the proposal/patches you have done
etc etc, we do have the ability to give access to certain sections of
the SVN such as branches without allowing committing to to the main
trunk of mediawiki (/phase3/))
Although some people may recommend doing major changes off site (eg:
github), I highly recommend against this since most of our devs don't
want to have to go offsite to review code changes unless you do it via
a Git to SVN gateway system/setup (eg: Brion does this for some work).