I think it was originally in September of 2018 that @Prof. Fartsparkle suggested to me right here on Hackaday.io to make a project like this as a "successor" to the #wESP32. Since at the time the wESP32 wasn't even released yet, that seemed a bit premature to me. :) But I liked the idea enough to do a good brainstorming session. I don't see it as a successor so much as a nice addition to the PoE products that are available to makers.
There are a couple of potential snags in making a PoE FeatherWing. The first is that the Feather system didn't intend for FeatherWings to back power the main board. That doesn't mean that nobody has done this tough. :) I think many Feather boards now include a diode to prevent back powering the USB port, and for those there is no issue. There are some that don't include this circuitry though, so the problem needs to be managed carefully.
I intend to do this by putting ~4.5V on the USB pin, instead of 5V. Since the flyback converter of the PoE supply has a diode rectifier on the output, it will just not provide power if there's a voltage higher than its own output on the USB pin already. That's the theory, let's hope it works in practice.
The second potential snag is of course size. Feather boards are tiny, and Ethernet jacks and flyback transformers are not. I have some confidence it can be done though. If necessary, I can put parts on both sides of the PCB, but I'm going to try to avoid it to ease manufacturing (although my CM keeps telling me it's a non-issue for them).
The plan at this point is to use the WIZnet W5500 just as it is used on the Adafruit Ethernet FeatherWing, so it is fully compatible with all the existing software out there. I will also add a 24AA02E48 chip to fix the annoying issue that WIZ5500 chips don't come with a MAC address built-in.
For the PoE side of things, this will have to shrink significantly from what I'm currently using on the wESP32. To that end, I'm going to build the design around the TI TPS23758 instead of the Silicon Labs Si3404A. The chip is slightly bigger, but it implements primary side regulation using the flyback transformer's auxiliary winding, making it possible to drop the huge opto-coupler and all the circuitry around the secondary side shunt regulator. I'll also be using a 5W instead of 13W flyback transformer as I have done on the #LiFePO4wered/ESP32, since Feather systems don't tend to be power hungry.