I encourage you to give scenario design a try. I generally find that picking a period or theatre and reading up on it focuses on how the armies performed and their strengths,weaknesses and tactics. At the other end of the medieval spectrum, War of the Roses would be a good arena for development of new armies and scenarios.
I don't know if you are familiar with BattleLore, a C&C medieval/fantasy game published around 2006, but it was full of ideas and innovations for the medieval period. Mainly it dealt with the 100 years war period, but could easily cover a range of situations, from fantasy, fiction to historical. Overall I prefer the C&C Medieval implementation as a more polished product, but BattleLore is a source of inspiration, and a fun game.