I'll be happy with the same model KSP had (an archive with data and binaries), almost*.
Flatpack or not, I'll be running any closed source app through bwrap anyway, including stuff in wine.
"Installers" are just a bad design spilling over from Window$.
Packaging may be too tedious given the number of distros, (although if it is just a bunch of files in /usr/local it might be super easy, barely an inconvenience).
* Now about that "almost". KSP didn't have the distinction between RO and RW stuff. It did not forbid mods from writing to the game's dir, and many mods relied on that. I do not know if the main game ever relied on writing into main game dir, apart from the config file. But anyway this is practically DOS-level disregard for file permissions. Whatever distribution model Linux version of KSA will have, I would very much like it to support residing in a root-owned RO path and writing to designated home and runtime paths. Conforming to XDG hierarchy would be the best way of course, but any simplified bare minimum would do.
I don't use Arch, BTW.