Exactly this has been on my mind for the past year except as a stand-alone key cap that can be placed on any keyboard. If anybody want to makes this a reality I would completely back that Kickstarter.
Power is a major issue for something like that, and you need to find a way to bodge it into the existing key matrix, or have it as it's own HID device (which I believe ruins combos). That and you don't have much space to work with either, especially if you still want to retain the full travel of the switch.
With Gergo [2] I took a different approach, and opted for every key to have pads for connection to the i2c bus, as well as a custom footprint for mounting devices (Needed as I2C is slow and that footprint is connected to a interrupt-able bus). That allows me to create stuff like PCBs cast in resin, vibrating keys and whatnot and whack them around the board, but there is a single slot for anything that needs quick feedback. When a board is 1x1.2cm, you only have so many options, and burying it in resin is valid.
But if you want to have a unified device, you pretty much need a firmware that can handle it. QMK is an amazing project if you've never seen it. It powers Gergo, drives the Trackball and communicates with my boards. And it's open source! Feels weird to be writing and flashing keyboard firmware, but it's crazy useful.
Threw a few more images here if you want to take a look at the module [1]
> Power is a major issue for something like that, and you need to find a way to bodge it into the existing key matrix, or have it as it's own HID device (which I believe ruins combos).
Make an own keyboard controller.
It also be very nice if it can actually handle trackpoint as an actual joystick (real trackpoint is a tensometer, not a position sensor.)
