Ubuntu’s popularity often makes it the default choice for new Linux users. But there are tons of other Linux operating systems that deserve your attention. As such, I’ve highlighted some Ubuntu alternatives so you can choose based on your needs and requirements—because conformity is boring.
They seem like different projects with different goals. Appimages are portable executables.
Flatpak, to me, is something you install on a system and run with a flatpak runtime that is installed on your PC. I think its a fantastic way to sandbox programs with differing dependencies, but you still install programs and run them on your PC.
Appimage, on the other hand, is a wholly-contained executable. It is less efficient than flatpak in every way if you are installing apps on a system, but it is more portable. I can throw a handful of appimages on a USB stick and carry them from machine to machine (or mount an ISO in the case of VMs). I can plug in my “troubleshooting and development” stick to an otherwise barebones server at my datacenter, fix an issue with a comfortable set of useful apps, then unplug and leave the machine untouched.
Appimage is not a replacement for flatpak, but it has its own purpose. Snap is more similar to flatpak, but inferior in every single way. If we must get rid of one, can we phase that one out?
Just be careful about trying to run your AppImages on a distro with for example only FUSEv3, because there are system dependencies.
I mean, in theory you could also put flatpaks onto a usb stick and symlink the directories. But nobody really does that.
But really, I think this could be a cool GTK app.
You would copy selected apps to the stick and include a program, maybe even with a GUI, that can then symlink those apps to the system you are currently using.