I have a few different suggestions.
1. I've been focused on earning as many stars as I could and it occurred to me that they don't have any in-game value. What is earned stars can be put toward unlocking upgrades or buffs for the support abilities like reducing cool down, increasing duration of deployment.
2. SmartGrid - SmartGrid is a simple feature that, when activated, will shift the tower placement outline over by the width or height of the tower being placed. This will make it easier to quickly tetris in towers next to one another without accidentally leaving gaps between them.
3. Quick Select System for both keyboard and controllers.
As an alternative to the hotkeys, hitting a button or key would open up a quick select menu featuring the top row of towers. The cursor by default would appear in the middle of the menu with the different tower icons appearing in a circle around it. The desired tower could be selected by moving the mouse or tilting a stick in its direction. The second set of towers would be in a second quick select menu and the support abilities would be in a third (and/or forth if needed).
4. Potential Controller Layout for PC/Mac and Consoles.
(By default each tower would be toggled to place multiples until placement is cancelled or a new tower/ability is selected.)
Quick Select Tower Menu #1 - left on dpad
Quick Select Tower Menu #2 - right on dpad
Quick Select Ability Menu #1 - up on dpad
Quick Select Ability Menu #2 - down on dpad
Cursor Movement - Right stick
Map Scroll - Left Stick
Place Tower/Ability - Right Bumper
Move Command - Right Trigger
Select Unit/Tower - A
Deselect Unit/Tower & Cancel Turret/Ability Placement - B
Upgrade Selected Tower/Building - Y
Change Game Speed - X
Activate/Deactivate SmartGrid & Sell Selected Tower - Left Trigger
Upgrade Command center - Left Bumper
Select - Rush Wave
Start - Game Menu
I believe all the control functions are covered here. If I missed something please let me know in your post.