I would like to see ground combat become more of a factor in production choices, resource allocation, improvement choices, and even the tech tree. Imagine being able to design legions of infantry, power armor, air assault, mechanized armor, etc with advanced weapons or make cheaper units to guard less strategic planets.
Then, taking it to another level, integrating space/ground combat with ship components designed to provide assistance to ground combat (Command and Control, Fire Support, Air Support, etc).
What if having an infantry school on an extreme world gave a bonus to units trained there on that type of world? Special Forces Units for Desert Warfare, Low Gravity, even Toxic or Radioactive environments. Maybe those units just wouldn't face penalties in those environments?