Consider importing bashism/kshism/zshism
Maybe some of them are useful in yash, too.
You asked for others so here is an idea. I think having ksh-compatible arrays would be useful. It is not uncommon to write semi-portable scripts that work on bash, *ksh and zsh's sh/ksh emulation, and use simple ksh-style indexed arrays. zsh has a 'ksharrays' shell option, you could implement one by the same name that would allow yash to run these scripts.