How to detect impending user touch before it happens?
For capacitive sensor check PIC12F1822/23 from Microchip.
You can also try an accelerometer as sensor. I created some device, connecting analog accelerometer module to ADC input MC and it senses even light touch.
Also you can try some barometric sensor or electret microphone. It will sense pressure change from air move.
Active sensors, like ultrasound and laser
may not fit your application because they have dead zone.
Infrared sensor like  AD8833 can be useful too, but it needs processing image.