I do not think what I would want as a replacement exists (yet). My main requirements are:
Only FOSS software and firmware
Similar level of “casting” compatibility/ubiquity as the discontinued Chromecast
Easy navigation and/or great UI/UX
Can be controlled with a stand alone remote control, phone/tablet/laptop, and remote services like Home Assistant
As portable and low powered as the discontinued Chromecast (or no less portable than a small mini-pc)
Ability to turn on/off the TV, switch inputs, and control the volume
Ability to install apps/plugins to directly on the device (maybe even things like Lutris, Moonlight, or something similar for gaming)
Ideally, the apps would be as well maintained and provide similar levels of quality as something like an Android TV or Apple TV
(bonus) Ability to store media locally for offline playback
I think the closest I have seen is LibreELEC + Kodi on a RaspberryPi or mini-pc. It’s still not quite there for my tastes though. Hopefully the recent Chromecast announcement will lead to more/better alternatives in the coming months!