Shape/component fill container (stack).
Example:

Let's say I have a horizontal stack with two squares and 20px margin between them. It would be really helpful if I could set one or both squares to fill the stack (container). This way If I want to resize the stack as a whole the two squares would automatically resize.

This does wonders for responsive layouts.
Figma has this feature already and it works great.

Love what you are doing with XD, I've been using it since day 1 and this is he only thing that's missing for me.

Keep up the great work and I hope someone reads my message :)