USB-PD is limited to 100W, so that's pretty much an instant non-starter. Doesn't mean they can't necessarily run TB3/4 channels over whatever data lanes the PSU ethernet port uses.It would have been really nice if the iMac was powered via a USB-C so that the Power brick also could have a Thunderbolt port so a big disk drive could be attached off desk.
This might be how the "mag safe + more ports" rumour for laptops pans out: a new magnetic charging port (shared with iMacs because thinness apparently), with a presumably high speed data channel, allowing the charging brick to provide various types of IO. I can't wait to hear how the whiners find a way to complain about it.