You're on the right track. Your first point is what we're shooting for -- keep the DC configuration entirely in host.xml. If the configuration gets too hairy and refers to too many things that would normally be specified in domain.xml, we'd have to reconsider.
Re: does DC need subsystems, it will for sure (at least optionally) need a web server to support the REST API. And I'm sure other runtime subsystems as well (threads, security). But that doesn't mean it needs the full configuration model for those subsystems. The configuration model/schema it exposes can be much more limited.