A whole lot of questions about the PandaPi, because it sounds awesome:
Looking at the images, I only see 2 hot-end thermistor and heater connectors. So it's OK for e.g. a Diamond 3-into-1 hotend, but not for 3 separate hot-ends?
What stepper drivers can I use? Obviously A4988, TMC2208, TMC2209. But what about DRV8825 (which I have several spare)?
Can the bed heater output switch 240VAC? Or would I need an external SSR?
You have Marlin 2.0 running as a Real Time task on the Pi CPU itself, right? So the only function of the STM32 CPU is to extend the number of IO ports? Seems overkill for a 32-bit CPU! Could you not connect to those devices (heaters, fans, thermistors) directly from the RPi via I2C or SPI? Especially since they are all slow speed and not particularly realtime-critical.
20A is a lot of current... the power connector doesn't look beefy enough. Is it rated for 20A?
Can I plug another RPi shield (e.g. watchdog timer, UPS) into the RPi?
Why not flash your MCU firmware from the RPi itself, rather than needing a micro-USB connector and UART?
Any issues with the RPi and/or stepper drivers overheating? I see you have big heatsinks on the drivers in one of your photos.
What are all the heater outputs rated at? I assume only the bed is 15A? Typically hotends are 40W, so 4A@12V or 2A@24V.
If I have a heated bed at 15A, and 2 hotends at 4A, that's more than the 20A you say the input is rated at.
Is there any need for an LCD and knob, with Marlin running on the Pi? Can't everything be controlled via the Octoprint website? Perhaps running on a full-size HDMI screen?
On the PandaPi3d.com website, I can't look at my cart with "View Cart" in either Firefox or Chrome, so I can't buy a board here. :(