The purpose of the transformation roadmap is to give a high level diagram identifying the changes throughout the transformation journey. The goal is to put an order to things, to highlight foundational components, identify dependencies and possible risks.
When putting this together I like to start with the the future state. Identify all of the big components and then mark them on a timeline including their dependencies. Continue this process breaking pieces down until each component is recorded. Try and mark dependencies and possible risks as you go along. The picture you are creating will help keep the transformation project on track. It creates a guideline.
Once you have the initial high level map identified, go back and start building out the first couple of steps. Ensure the first couple of steps are achievable given the time period and resources available.
Always keep in mind it is the EAs job to identify potential risk, dependencies and to educate the executives. The executive are the ones that make all of the decisions, having said that I always come to the table with my recommendations in mind.
This roadmap will be used during the yearly business planning process and throughout the transformation journey.