Why when we talk about "editor engagement" we think exclusively about new editors? How about retaining people, who already made Wikipedia (= the product) and keep maintaining it?
Retention of people who have made dozens of edits is about the same as it's ever been. Retention of people who've made a handful of edits has declined substantially since 2005, even though new users still show up at the same rate.
I believe sister projects are deeply important....
All of them have much more popular alternatives doing the same thing they do. For example, PeerWise is vastly more popular than Wikiversity, and is being integrated into thousands of existing institutions' courses far faster than the http://peerwise.cs.auckland.ac.nz site can keep up with. On the other hand, the many commercial Commons alternatives are doing fine on their own. It would be more appropriate to reach out to existing non-profit, wiki-like organizations such as PeerWise and simply offer them hosting support than try to pour resources into Wikiversity.