Do you mean that the numpad and battery packs would
slot onto the main unit, making it one larger rigid unit?
As a use case: when you're home, you would just use the 65%+numpad and a USB cable. But when you're travelling, you could leave the numpad at home and bring a battery pack instead.
For a perfectly flat keyboard, that would be interesting. I prefer flat, low keyboards myself (with full-travel mechanical keys mind you!) ... but I often feel like I am in minority in liking flat. Most keyboard are raised in the back and then there would be more than enough room for a flat LiPo battery there. With a modern microcontroller with wireless capability and firmware written to conserve power, you should be able to go a long time between charging, obviating any immediate need for the battery to be in a swappable unit. As long as you can open the keyboard up and replace the battery after a few years.
Modular keyboards have been done before. Most older ones use cables... but
* Sidewinder X6 has a numpad that can be plugged onto either side. It uses angular shapes, magnets and some proprietary connectors that remind me of USB connectors (plugs on the main unit, sockets on the numpad).
* The "Ultimate Hacking Keyboard" uses rods and pogo pins to connect left and right halves.
For microcontroller, I would look at the Nordic Semiconductor nRF52840. It supports USB and BLE and there are now several microcontrollerr boards out there with it, and more and more DIY keyboards that use it.
One microcontroller board is the
Adafruit Feather nRF52840 Express, which is able to charge a LiPo from its USB port. It does not expose many of the µC's GPIO pins but the schema is freely available for inspiration!
BTW, I think more keyboard kits for the DIY'ers could be more modular in their parts, so that you would be able to choose left or right numpad
when you build it. The USB port is best broken out to its own board either way.