Some suggestions from me:
1. Making native tribe units more relevant, or just the native tribes in general more relevant. They were supposed to be one of the core features of the game after all!
2. Making healers, surgeons, and field hospitals actually useful.
Both of these things are almost never used, and often the payoff is poor. More often than not, using them is punished. Both of these things are HUGE parts of the game that are just on the side, collecting dust. The potential of these features is huge! They just need to become viable and worthwhile.