1. Using a tool that allows you to build using OBS once, and distribute for all distros is already a solution that makes it possible to target all of Linux.

There are projects using this method, but bigs like BlackMagic would prefer shipping one package (like he does right now with DaVinci Resolve). Anyway, after installing a package downloaded from a site, how do you update it? Who publish that software should make a repo for every package type or making app update itself (like apps on Windows do).

Flatpak's sandbox isn't really doing much for security/privacy as addressed by this: https://flatkill.org/2020

Thanks for the link, I was aware of those issues but wasn't aware of this website. Anyway, the major issue here is old bundled libraries, with further spreading of flatpak other issues should be trivial to fix, I hope.

we need to figure out how to get less permissive, well, permissions, to applications

Libportal should fix this.

as well as to apply system theming by default

My flatpaks apps follows system theme by editing global vars, there are a bunch of guides to do it. Distros could add them by default, but (as you said) theming is still controversial.

AND YES, this post was mostly an experiment to see what people think and how they'd react to differing opinions different from the status quo.

Next time just ask. Would make more people engage in commenting rather than just downvoting.

having to type the whole thing. What I mean is running "flatpak run one.ablaze.floorp" instead of just "floorp", for example.

That's to avoid conflicts, flatpak install looks up for entries that's why you don't have to write the whole thing.

Flatseal. I mean, Flathub has THE control center for Flatpak apps and nobody has taken it upon themselves to make this more official

What flatseal does is giving a GUI for configuring flatpaks, you can just use flatpak command itself from cli (that's the official way). That should be embedded in system settings (gnome-control-center for gnome).

Also for Flatseal specifically, can we make it easier to theme (gtk and qt) apps, (like a dropdown or something?) instead of having to look up the envvar name because I can't remember it?

This is entirely feasible, try ask flatseal devs by opening an issue.

Can we find a way to force apps that don't really need full filesystem permissions to remove that?

Maybe like Android does: first time you open an app it asks you to grant permissions to that app without giving them all the permissions it asks by default. That way you can just opt which permissions would like to give to an app on installation or first launch, tho this is not what happen right now because can entirely break some apps so it's up to power users to tweak it.