Thanks for the suggestion. This is quite tricky and it needs a lot of testing to make sure that it does not break anything since the bidi package current approach has existed for at least 2 decades and I do not want to break anything for users.

From memory, I was in the past 10 years was thinking of doing something similar; at the time, the package hopatch of Heiko Oberdiek existed and I experimented with it but from memory, it was not working in some cases (sadly, I can not remember those specific cases/examples).

Having said that, I willl look into this when I have time and will release an experimental version so people can test.

It should not affect old documents, as all the current documents that use bidi have to load packages before bidi is loaded, and in that case the all the .def files will be \input'ed at the same order as they use to.

But my first suggestion wasn't considering catcodes:

\newcommand*\bidi@restore@catcode{}
\newcommand\bidi@isloaded[1]{
      \@ifpackageloaded{#1}%
            {\input{#1-xetex-bidi.def}}%
            {\AddToHook{package/#1/after}{
               \edef\bidi@restore@catcode{\catcode`\@=\the\catcode`\@}
               \makeatletter
               \input{#1-xetex-bidi.def}
               \bidi@restore@catcode
        }
    }
}

It should not affect old documents, as all the current documents that use bidi have to load packages before bidi is loaded, and in that case the all the .def files will be \input'ed at the same order as they use to.

It is not that simple.

It is not that simple.

I know you are currently very busy, but if you can elaborate a little bit more, I'll be happy to learn how could that potentially affect already existing documents.