I'm still a huge fan of Ventoy, but lately I have been finding more and more issues with it.
So I decided to investigate using a Raspberry Pi Zero with a USB adapter to create a virtual drive:
It's very wonky and manual at the moment, but I have managed to boot all Linux ISOs successfully so far. Unfortunately, I think only ISOhybrid works OOB, so Windows ISO do not work. I have found some scripts to take Windows ISO and make them ISOhybrid, but haven't gotten around to doing that yet.
I think it should be doable to package this nicely.
My crazy idea is: write software so that Flatpaks can run on Windows and macOS. Plus, make high-quality Flatpak-building templates available for as many programming languages, UI toolkits, etc. as possible.
Because everything that Flatpaks provide is OSS, making shims for Windows and macOS compatibility would be tedious, but doable.
Same with crosscompiling Flatpaks, compared to the difficulties of crosscompiling for Windows or macOS from any other OS, multiplatform Flatpaks should be doable to crosscompile.
So this would lead to a world where a very convenient way to package for Windows and macOS... is creating a Flatpak that works on Linux!