There is no plan to release a separate OmniFocus Pro app.
When OmniOutliner was released on the Mac App Store, in-app purchase was not available yet (as I recall). So we had to release separate apps, and people had to choose up front which edition they wanted. That was rather confusing. We think it’s much clearer to offer a single app, as we’ve done with OmniGraffle and now OmniFocus.