Definitely hotkeys. Sell and upgrade/buy NEED hotkeys. When I've built a row of twenty turrets, I then have to swing my mouse up and down twenty times to upgrade them. This takes more than twenty times as long as clicking each turret and tapping a hotkey three times.
Hint: Both S and U are unused, as far as I can tell, as well as B. As is A. ( [A]lternative upgrade?) You can also, to cover the laser as well, use ASD - it's not intuitive, but it uses three keys in a row, for three options. (Sell, upgrade, upgrade to AA laser.)
Need more of those alternative turret upgrades.
EMP turrets. (Pulse emitter alternative upgrade?)
Base repair purchases. Possibly HQ upgrades for global effects. Added range and such, perhaps upgrades or unlocks for bomb/emp/nuke.
More powerful AA weapons. Or weaker air forces. Or stronger land units. Air units ALWAYS end my game.
More game modes. (Sandbox, to test mazes and turret effectiveness, etc etc.)
Lastly, range bonuses are almost worthless. I don't mind if the low-tech towers have shorter ranges, as long as the range bonuses are actually worthwhile.