Integrated powerline networking
It would be fantastic if eeros had powerline networking built right in. Powerline networking is cheap, simple, and extremely effective. This enables you to have wired extensions instead of wireless, which are faster, more reliable, and not subject to interference.
Having a separate device taking up another plug and a wire between it and the eero is unnecessary - this functionality could be entirely built into the Eero or its power brick, so your eero mesh could be extended over your home's power lines completely organically, with no additional devices.