Choosing a microcontroller and application for HID

I have a project idea that I want to implement and I know it will require a microcontroller. I want to create an HID device that'll most likely simulate a keyboard more than anything. I want to use/create an application that can program the HID reports to keyboard commands e.g. send the letter K if HID report = 0x3023 (or whatever lol). I understand that I can use Teensy device, but I want to learn more about MCs and how to program them.

What MC should I choose for this simple application? Is there any project that implements the idea of programmable HID?