I would find useful an option to send the original resolution of each video to Apple TV without upscaling. There are starting to be TVs on the market that do upscaling with the help of artificial intelligence and more will come in the future. So it would be good to have the option to send to the TV, via Apple TV, the various videos in their original resolution leaving only to the TV this task.
The Apple TV does not allow dynamic changing resolutions as far as I know. Everything is fixed. Honestly the upscaling on the Apple TV is pretty good today. You could try forcing the output to hd and have your tv do it and compare to see if you prefer one over the other.