I disagree with these two things:
- having only a network-wide Meta site, and
- having the Meta-site on a sub domain.
To 1
I think having only a network-wide Meta would result in overlapping interests and actively harm community building. This is, because Meta is the place where the community is shaped and it should be a place dedicated specifically to that single community.
Furthermore it should be easy to join and to participate in Meta, as we strive to build a community-run platform and Meta is, where community governance begins. Having a separate system will lead, with a high probability, to a smaller group of people being active on the Meta site and having a lot of influence to critical decisions. This is bad for community governance.
To 2
I think we should not simply use a slightly modified copy of our main Q&A platform to host Meta sites. This is a bad idea IMO, as Q&A systems are not designed for discussion. I think we have the chance to build a better alternative to the current Meta design of Stack Exchange.
Post talk pages can be one part of this, but should be accompanied by a instance-wide discussion software for policies and that kind of stuff.
We have the chance to build a Meta system, that is integrated into the site, from the beginning. This can make things way easier than the current system, which was an afterthought AFAIK.