When you begin a new save file, all the missions that can be selected are green (level 1) and short. Gradually, I've noticed more and more medium and long missions as well as yellow (level 3) missions.
What triggers these missions showing up? The possibilities I can think of are:
- Average level of your heroes
- Highest level of your heroes
- Number of weeks passed
- Number of heirlooms collected
- Number of missions completed
- Number of bosses defeated
Finally, is there a point where level 1 missions no longer appear?