Larger Display Module
1.9 inch + make the "wheel" a real potentiometer that can do stuff. The current PCB is already set up for this.
32-round Magazine
Based off the CP33 geometry
Magnetic I2C Connectors for the "Front" and "Back"
This will make sliding the handguard on/off not such a huge pain.
Solderless build
This may be impossible, but at the very least the goal is to remove as much soldering as possible.