CORE-818 and CORE-819 address the domain management plugin (These Epics are currently private). We are working out the details internally, and considering everything everyone has had to say regarding domains. It's likely that we will open these tasks up and start a discussion in the near future.
These features will be dependent on 4.0 however, which we want to get out relatively quickly. There are a lot of major benefits to developers in 4.0, in part because of the PHP dependency change to PHP 5.4, namespaces, and more. Domain management won't be in 4.0, but we are continuing to make progress in designing the feature, and implementing some other core changes so that it will be easier to implement. (For example, plugins will be able to control package pricing. This allows for the domain plugin to set pricing for domains (Think pricing grid for new/renew/transfer, 1-10 years). It's a powerful feature that could be used by other plugins for a variety of purposes.)