I have proposed an idea of Belt buffer device and at the same time Klonan has already developed a mod with the same functionality
https://mods.factorio.com/mods/Klonan/belt_buffer
This is what belt compressor could be (graphics aside)

Even though this is awesome, I would either want this item ...

Great work with the update, thank you devs!

On belt compression:
I personally would like to make belt compression as easy and intuitive as possible, because
a) I would like belts to stay competitive to bots as long as possible in the late game
b) it is an unnecessary hurdle for new players to ...