The cross-compiler part itself is easy, but getting all the build scripting of tens of thousands of Fedora packages to work perfectly for cross-compiling would be a lot of work.

There are lots of small issues (libraries or headers not being found, wrong libraries or headers being found, build scripts trying to run the binaries they just built, wrong compiler being used, wrong flags being used, etc.) when trying to cross-compile arbitrary software.

All fixable (cross-compiling entire distributions is a thing), but a lot of work and an extra maintenance burden.