Being able to use commands provided by some common LaTeX packages with the "native" mode of the Math extension of MediaWiki would be very beneficial. The packages I have in mind (apart from the obvious amsmath and mathtools) are physics and siunitx, from which most of the commands are relevant.
For instance, being able to simply write \ket{\psi} is a very basic functionality for any quantum physics related article, and the alternative \left\vert \psi \right\rangle very time consuming and error prone. Same goes for \pdv{f}{x}{y}. Having an implementation of common packages also allows to copy equation from personal drafts without time-consuming rewriting, hence easing the contributions.
To go beyond the packages I propose, the selection of packages implemented in Mathjax and LaTeXML is relevant.