Relational positions of items within the page
I suggest to add the ability to make OBJECT 1 placed in relation (left - up - down or right ) to OBJECT 2 . Object 2 can be a a text, shape, group or the artboard itself .
For Example : I need OK button located always on the right bottom corner with 80px margins. When I extend the size of page (vertically or horizontally), OK button will remain at the same position.
You can already pin objects to the page by setting responsive resize to “manual” and setting the pin. For more information, please check out this page: https://helpx.adobe.com/xd/help/using-responsive-resize.html#Manuallyeditconstraints