Good Idea in theory, but i think it would fall short because

• supporting multiple runtimes requires more develeoper effort, if even possible
• a single app is enough for a runtime to be required to be present. So it's unlikely to save space
• The number of possible runtimes isn't that high anyway. I think the flatpak folder on my pc is like 10GiB.

Sounds like a temporary problem.

I don't quite get what you want to achieve. On Android Apps might run Tasks or be active in the background. But on Linux programs usually dont have a background task, if you close them they are gone. (Unless they "minimize to taskbar")

Then, when I kill the process, have it automatically lose all privileges and become "frozen".

So, when you kill the process it's gone. Removing permission won't change anything for not running programms.

Maybe you can explain a bit more?

I do, i dont want to have to access 5 accounts using the browser on 3 different websites

Unfortunaly protonmail is not possible local (afaik) so i have to check there in the Browser.