Microcontroller Dev Boards: What features do you like to have (or to add?)

At uni I used this microcontroller dev-board quite a few times:

 (Image source: Google Images)

the play area on the right was really handy for making projects. There wasn't really much else that the board did. It had built-in RS232 for programming or comms. It had an option for adding a real-time clock. I really liked it so after I left uni I made my own version of this (on proto-board with tons of wiring,no PCB) but with the microcontroller replaced with a version that could have external memory (I think I used parallel EEPROM). That helped loads, since otherwise you had to run code from RAM (which was limited to 512 bytes), or use one-time programmable ROM, or EPROM window versions.

I learned these suggestions that suited me, but might not suit everyone, when working on the Pico Eurocard:

1. Keep it simple
2. At least two connections per pin, using pin headers or whatever your favorite connectors, makes it easy to simultaneously probe. Try to make it easy to access buses like I2C, SPI.
3. Examine the chip datasheet, and see which pins are are least likely to be needed for SPI/I2C etc, and use them for some sort of minimal I/O like buttons and LEDs, but use jumpers to disable or re-patch them to other I/O if needed
4. If there is no USB serial debug capability on the microcontroller, then strongly consider adding at least the UART connections onto a pin header, laid out to fit your favorite USB-UART adapter, or better yet, solder one permanently on, since it helps troubleshooting immensely if you can use print statements in your code. I also like creating a keypress entry menu, so that code can be tested without requiring buttons/displays
5. Use the ARM SWD or JTAG connections (or whatever the microcontroller supports) because it can often be much quicker. This is easy with the Pico becase the debugger happens to be another RP2040 in that XIAO module of course.
6. You can't please everyone, everyone will have their preferred displays and connectors and so on, so consider the board as a kind of test card for the processor, and just have plenty of connections for easy extension. In other words, keep it simple.
7. If you're creating a prototyping area, then if you deliberately don't make the pads through-hole, then components are extremely easy to desolder, since the copper won't quite reach the drilled holes, it will automatically be set back a bit, with all low-cost prototyping services.
8. If you use an ecosystem a lot, like MikroBus, then you could either add the footprint, or alternatively make yourself a carrier card/plug on board for it.
9. Since it's a dev-board/test card, it doesn't need to be ultra-compact. Consider 100x100 mm as a starting point, since that is often a low cost from the PCB services.
10. Since the user will often need access to connections, it often rules out a plastic enclosure, but instead consider either mounting feet, or use screw-holes to attach a plastic plate on the underside and top. The latter is usually an excellent idea, since you don't want a stray wire on your desk to touch something.

I'm also finding the built-in supplies really handy. I like working on laptop on sofa or wherever, and it means I can just use any DC power supply, and have access to 5V or 3.3V rails for circuitry. I'm using the supply feature a lot (although the Pico is unusual in that it is fairly generous with what it can supply). 

I'm also finding the built-in supplies really handy. I like working on laptop on sofa or wherever, and it means I can just use any DC power supply, and have access to 5V or 3.3V rails for circuitry. I'm using the supply feature a lot (although the Pico is unusual in that it is fairly generous with what it can supply).