We are trying to build a system with reusable plans - where a plan is essentially a step-by-step instructions plus timing data. So the idea is - somebody creates a good plan - like a recipe, or first 10 weeks of life of a child - and then many people should be able use it.
So master plan should remain the same, in our opinion. On the other hand, you can change positions of steps, add/remove reminders and if you are not satisfied with the plan completely, you can duplicate it and create a new one based on existing.
We are not trying to create yet another project management tool - it is more of tutorial with timeline, reminders, images, videos and reusability is the main focus for us.