Evolution is cool, however it tends to just be basic stats. Consider two avenues. Making the enemies stronger across the board to match the player's success. But separately changing tactics/mutations based upon the player's tactics.
- If during a mission there were X number of turns that three or more phoenix point members where within grenade range, include more explosive mutations.
- If a preponderance of single shot weapons are being used, trade a lot of health for little armor. (Alternatively in response to a lot of armor piercing)
- If multi-shot weapons are being used increase armor for a reduction in health.
- If status effects are being used, increase regeneration at the cost of health
- If explosives are being used, spread out. (Both deployment and during movement)
The concept is that all of these things are based on the last X missions that Phoenix Point takes and continuously change throughout the game. (Note this may be global or limited by a lair/citadel_