Would it be worthwhile to support UART for a serial 'plug-n-play' interface? Or would it be a waste of time?
I would say to support a UART. Almost all MCUs have a UART, and as @tcrosley said, many comm devices have UART interfaces.

I've done similar designs where I've added provisions for daughter-boards on servo controllers without knowing what they might end up being. It's not always easy to plan ahead. I recently added Bluetooth to a servo controller as a daughter-board, using the UART interface, so I was lucky that I had the UART pinned-out.

What @tcrosley says about "plug-and-play" is very true. Almost no interface that would be appropriate for a daughter-board lends itself to "plug-and-play". But since you likely have control over the creation of the daughter-boards, there are simple solutions.

My favorite solution is based on the fact that almost all the MCUs have an ADC. I dedicate one pin on the daughter-board interface for "board-detect". I wire that pin to an ADC channel that has a pull-up resistor. Each different daughter-board has a different value resistor assigned, and wired as a pull-down. The resulting voltage divider delivers a "signature" to the MCU.

The MCU reads the ADC on power-up, and then knows which daughter-board is attached (if any). Of course, the MCU would need firmware for each different board, but that would almost always be the case.