An idea i had to keep the game feeling fresh and exciting is for additional rewards for completing either regular missions or top priority to earn points for a skill tree esque mechanic
Let's say you complete a top priority mission and you earn an AP (advancement point), and you feel like you run out of sprint too fast, you can apply an AP to stamina, so you can sprint a little bit longer before needing to take a break.
Or health points, you can upgrade your health so you can tank more hits before needing to duck and heal.
Or what if you want sharper aim without taking more than a few seconds to get your sights dead on while taking fire, bullet tracking (or whatever you'd wanna call it) adds a very slight, very subtle magnetic like effect to where if your aim is close but not close enough to hit, your bullet will shoot towards the target instead of straight forward.
And what about that factor of having shaky arms or just can't aim that far without swaying about? Stability (from ITR1) is a one-time upgrade you can get to gain the option to hold or click a button when you're aiming down a scope to eliminate sway.
These are just a few ideas i had while playing.

