think about importing all current user logins/homepages/... to the new site (possibly with an "opt-out" option) - that way the hurdle to go to the new site is low

No, not just import, have a shared login mechanism, like OpenID.

And then, have XP/Reputation be shareable.

And have "Recent Threads", "Newest Nodes" on each site be an RSS feed

And learn a bit from federated social networking systems- crabgrass, diaspora, onesocialweb, etc- on ways to integrate those site partions that should be unified.