I've been working on this function that makes something similar to LaTeX's \nicefrac:
#let over(a, b) = $#h(0pt)^(#scale(a, 140%)) #h(-1pt) slash #h(-2.5pt)_(#scale(b, 140%))$

$over(a, b)$ yields:

My function is very hacky and should instead be implemented properly. Following is an example of where it fails:
$a/(over(b, c))$ yields:

It would be even better if the behavior of the / operator was configurable in the template! i.e. the user could switch between the current default, a literal slash, or a nicefrac-like result.