Thats a very good question, and something more people should be asking : here is a terse reply : adopt a part of the distro, contribute tests and take ownership of driving support for those components forward ( so, wiki content, support in irc channels and support for users on those components in the mailing lists ). Start with a package or two, then move that forward. Start with whats already in the distro.
Its easy to fixate on the idea of CentOS being the distro and the distro alone - however, a very large part of what the users see value in is the user base around CentOS - and focused, specialised help with those areas would go a long way in 'helping' CentOS.
Would this help get a future release out of the door quicker?