I make heavy use of grids for tiled artwork, and I would like to see the ability to have subdivisions within the grid, OR to use a different colour for every Nth grid line. Highlighting every Nth grid line is probably the most practical way, since it doesn’t require redefining what “snap to grid” means, and ensures that all grid lines occur at pixel boundaries.
This would be useful for drawing lines at specific angles other than what Shift+Line Tool provides while still keeping a visible tile grid, for evenly positioning details within large tiles, and for navigating large gridded canvases more intuitively.