Ability to set zero-point or grid offset for Artboard grid
The current Artboard grid does not allow for offsetting. This means if you wish the grid to start at a particular x-offset or y-offset you are out of luck.
e.g. if you are designing UI for the iPhone 7 at 375pt width, and you wish to use an 8pt grid, 375 does not divide neatly into 8 and therefore you may wish to add a left margin of 4pt and a right margin of 3pt. Also, you may decide that you wish your 8 pt grid to align vertically to the text in the status bar, so wish to offset the y of the grid to make this work.
The way this is handled by InDesign and PhotoShop is to allow the user to drag out the 'zero point' of the rulers (and by association, the grid) to an offset of the user's choosing.
I also think, the ability to specify subdivisions to the grid i.e a 16pt grid with 3 subdivisions creates a sub-grid of 6 pts, is of high importance also.
Yep, it's pretty limiting that the grid only starts from the upper left of the document, with no options to center it, etc.