I am a big Omnigroup fan, my compliments to your products.
Having said this, I make use of OmniPlan and the app works well. Recently I purchased the app Roadmap Planner through the app store (see also https://www.keepsolid.com/roadmap/). Although this app is far from mature it works like a charm when it comes to quickly creating a planning and I am using it in a project where both Prince and Scrum are combined.
You should really have a look at its potential in relation to OmniPlan, it would make a great enhancement to the already wonderful functionality you offer!