I often find myself working with multiple layers which all have the same set of expressions applied - eg, one for position, one for rotation, etc. Sometimes there might be one expression that I have to paste between layers in multiple different comps, and then if I need to add extra functions to that expression I've got to copy and paste it on all the layers again. What would be useful is a global "Expressions" object - basically a panel where you could define functions, then refer to them from anywhere:
[in the Expressions panel]
function calculatePosition(p, i, dx) { // p is the object's current position, i is its index, dx is x-spacing x = p[0] + (i * dx); y = p[1]; return [x, y];}
[in a layer's Position property]
Expressions.calculatePosition(transform.position, index, 200)
Then if you decide you want to space things, say, vertically rather than horizontally, you just need to change one thing.
I've submitted this as a feature request, but I don't know if it'd be useful to anyone else - it's not in the list of possible upcoming features as far as I know.