My $.02 here is that I would tread very lightly here. I like to see as little data as possible stored in the database, as opposed to embedded in the files.
One of the biggest strengths I see for PM+ is that all your data is embedded safely in the files. You don't have the Lightroom database of Damocles hanging over your head. If the database dies, there's no big worry, just trash it and rebuild it. (Apart from loss of the collections, and, frankly, I wouldn't mind seeing them stored in separate flat files, like Snapshots.)
Interoperability with other programs would take a hit, too. If users are storing their assets in Photo Mechanic's DAM, that means by definition that they are using something else as their processing application. I think it's critical that when an edit is made in any part of Photo Mechanic, Plus or not, that it should read in any other program without requiring any machinations in Photo Mechanic. (Notwithstanding that, depending on the destination app and file type, there may or may not be a command to issue there to read the edits.)