I'm currently considering writing up some macros in order to provide a somewhat more reasonable alternative to what the
physics package does. This is heavily inspired by Alternatives to the physics package.
The problem is: Which kind of macros should be implemented? I don't want to add questionable syntax like the package does, but more reasonable things with proper LaTeX typical optional arguments or even some more unconventional but still (imho) reasonable things, e.g.
_ starting an optional argument for something that should be subscript and right delimited by either a
^ (starting a superscript) or the next mandatory argument.
Everyone is encouraged to wish for macros I should implement. If somebody has a finished solution for something that should be in the package -- and the person is willing to share it -- one can post those solutions as answers as well.
So what can a single answer to this question contain? A wish for a macro (optionally along with syntax proposals) or a macro-implementation (which might be subject to further discussions). Please only one per answer. You might upvote implementations or macro-wishes you deem a good idea to be in such a package.
Edit: So far the packages
diffcoeff were brought to my attention, which seem to provide macros for subsets of the
physics package's functionality.