Ability to use same mask for multiple layers.
... In practice if you change the mask, all the instances of the mask changed as well.
This feature combined with automatic reloading of external mask files would make Substance painter really powerful as it would enable users to use other software (for example photoshop) for advanced mask creation and see the results nearly in realtime.
Thomas Schmall commented
There is also this feedback that makes the suggestion to make the whole stack system node based: https://allegorithmic.uservoice.com/forums/261284-substance-painter/suggestions/8405988-node-system-for-layer-composition
It just makes more sense and would be way more flexible (i.e. reusing masks and materials).
Same for the brush system (made the feedback fro that: https://allegorithmic.uservoice.com/forums/261284-substance-painter/suggestions/13884003-node-based-brush-engine )
Just came across this today. It's alright to copy paste the masks to different layers, but being able to instance them or link them would be a great time saver.
This could work well as an mask/content layer effect. Like a fill effect that just copies the contents of a mask / content layer from the same project and gets updated if the source mask/ content layer changes.