If the original package is dead, you might be able to convince the AUR maintainer to switch to the fork.
In other cases, maybe a package with
<main feature>
added in the name could be created. I'd posit Supersonic as an example, where alternative packages exist just for enabling Wayland. It's still the same upstream, but an additional option in the build is required to enable Wayland.
Similarly, Emacs offers multiple versions with different build flags, though in this case it is using the same PKGBUILD for all versions.
Of course it's Nanowar of Steel. Awesome!