I only play on commander, why my suggestions are related to that diffculty.
1. Aircombat
The player should not know a specific UFOs range and fire arc before they shoot one down and "bring it back" for research. It will also be easier to detect a "new" UFO in the airgame as it lacks the highlighted fire arc areas. That might be hard to code, but it would be more realistic.
I find the airgame too easy in the beginning. I mean a single fighter should not be able to bring down a Destroyer with a couple of sidewinders and an autocannon.
Otherwise I love the airgame, it really add to the the game as a whole.
2. Shield
I find the shield to be a bit overpowered. Maybe lower its HP to 70/140 or even 60/120 on commander. I believe it will be more of a tactial gameplay with lower health on the shield. You can no longer stand in the open just tanking all the shots.
3. Knife
An upgraded knife with technology would be interesting. For us who like to go melee. Nothing major, but maybe a laserknife!
It is a great game, looking forward to the endgame!