When during the course of a project, should you incorporate temporal features into the design of the database? Should a roadmap be developed for projects to follow?
I am not pretty sure of your question specifically. However, let me answer it this way.
It depends on the project specifications and end requirements. It can be at ODS, EDW or Semantic or even source.
Temporal is a physical database desgin concept and you will have to decide at design time which table will be designed using Temporal feature.