U(S)ART/SPI/I2C - any common/prevalent/recommended connectors?
I²C and SPI are typically used as an interface between chips on the same circuit board.

"UART" is not a complete interface standard.
There are several such standards based on UARTs, such as RS-232, RS-422, RS-485, and MIDI.
But for a general-purpose UART, you don't know which one to use.

An "USART" is an UART module that also supports synchronous communication (usually SPI and I²C).



Without a single commonly accepted variant, the most flexible method is to use a 0.1"-pitch socket strip or terminal strip, and force the user to make the connections manually.

Again, there is no single standard for which signal goes to which pin.
You could make the entire connector compatible with Arduino, or Raspberry Pi, or LaunchPad, or any other microcontroller board, but then you'd have to choose one.