You might be surprised by how few GPIOs are used in e.g. contactless touch screens - they detect fingers using a sparse grid of X and Y wires - simplified - a finger is detected near 2 or more "X" axis wires then detect finger near 2 or more "Y" axis wires.
Basically the proximity of a nearby finger changes the capacitance to free space of a wire , and the microcontroller cycles driving lines and switching them to an ADC to measure the way the wires charge and discharge over time to measure the capacitance.
So a grid of 8x8 wires might be able to detect a finger in more than 64 positions relative to the wires.
Some microcontrollers e.g. STM32L152 family have convenient analog multiplexers and switches designed for touch switch driving.
It may be simpler to consider continue using a Theremin style approach to musical instrument interface .. You dont need 68 oscillators, your finger affects several at a time, so maybe 10 oscillators (but then there is "injection locking" where they will mostly run at the same frequency because they can "see" each other.. )
Or find a resistive touch screen monitor from some old point of sale terminal that has not been destroyed by the users, and the resistive touch screen sensors are easier as you only use four wires to drive them ..
Or use a camera or two that looks across the keyboard, works out where your fingers are.
Or use a matrix of infrared beams that your finger interrupts (like my old Sony E-Reader touch screen : one LED is "seen" by several receivers on the other side of the screen, a finger or a stylus gets in the way of the path of the infrared to one or more receivers. Change the LED thats switched on and the pattern of shadows moves.)