I would love to be able to define new LaTeX commands. This could be implemented as a user-defined preamble (text settable in settings) that would be injected to MathJax on every page.
Why I want it: Because I don’t enjoy typing $$\mathbb{R}$$ and such whenever I want to jot down a quick note at school.
The preamble would just need to go into a display: none
div at the beginning of the document (inside LaTeX marks, of course). I have done it here: https://github.com/AnotherKamila/poznamky_ma/blob/master/Makefile#L10
Slightly OT, but it would also be pretty awesome to enable AsciiMath in addition to LaTeX. MathJax supports it.