FYI, AppImages do not run anywhere. There are a lot of issues with them in NixOS, since NixOS is all about having explicitly-linked dependencies, and AppImages still often have implicit dependencies that aren't in the image itself, since they are assumed to exist on the host system.
Yeah, AppImages are not really comparable to snap and Flatpak, which both address this issue by managing shared sets of libraries as well as applications. Flatpak calls these sets of libraries "runtimes", and snap calls them "base snaps". The metadata for each application declares one runtime or base snap that it uses.
This ability to easily provide different userlands to different applications is one of the critical advantages of these new application formats.
and, before that, put all the missing dynamic libraries in the same path. That task might however be nontrivial due to (hiearchical/nested) dependencies (try "ldd *.so" to inspect it) of the so files itself. That's one of the reasons why we have package managers at the first place...
See https://github.com/NixOS/nixpkgs/pull/51060 for an example.