It sounds simple, and in theory, it is, but in practice, it isn’t – there’s a huge amount about this to get right, which is why we say yay for the likes of SparkFun, Adafruit, et al., who make things for us.
Benjamin Shockley has created his own line of ARM Cortex boards that you can program in either Arduino or CircuitPython. More important than that, they show that when you’re not trying to maximise manufacturing efficiencies, you can make boards any shape you want.