I just wanted to let you know that we're actively working on the development MAEC 4.0. The primary change from 3.0 will be the incorporation of CybOX v2.0 (http://cybox.mitre.org/language/version2.0/), which was released a few days ago. Thus, while we had hoped to make this a minor revision, the scope of changes in CybOX v2.0 means that backwards compatibility with previous 3.x versions will not be possible, and thus a new major version is necessary.
The ETA for this release is by the end of the month, but we'll keep you posted with more firm dates as we make progress on the release. The ALPHA versions of schemas, with the CybOX v2.0 integration, are currently up on our schemas GitHub repository, and will be updated accordingly as we add the new features and make tweaks: