Please add major grid lines
GIMP has a most excellent grid feature, with an ability to draw a wide variety of different styles of lines. But the one thing it does not have is an ability to add "major gridlines," which is, to have have every Xth line appear different than the others, usually darker or thicker.
This feature is very useful for tracking content when working at various sizes, as the user can see an indication of about how many grid squares a section occupies when they are zoomed out too far for the grid to be visible/useful. It is also useful for situations where a user is creating art where the user needs to aware of how it fits at multiple values, for example creating pixel art where the tiles need to fit into an 8x8 grid for individual tiles and a 16x16 grid for placement in a map.
In its simplest form, we could just have the "Configure Image Grid" dialogue add a "major gridlines" section, which would include a checkbox (to enable/disable the feature) and another number field to indicate how many gridlines are between major gridlines, and then the same options given in the "appearance" section.
A possible more advanced form might be to allow multiple grids, each with the full set of customization possible in the current grid options. This would allow users to stack multiple layers of grids to create more complex grid systems. They could create an additional tier of major gridlines, have major gridlines that are offset in different values between X and Y, or even simply have gridlines that get rendered when an image is zoomed out further. That being said, I personally don't think it needs to be that complicated and I personally don't forsee having a need for that much control over my grid, but it doesn't sound completely out of the realm of possibility for something that someone might want.
I also want to mention with this feature that if it is implemented the current grid system should have grids disappear sooner when a user is zooming out. One of the primary uses of major gridlines is to preserve a sense of scale when zooming out, so the smaller sets of gridlines simply have less need to be drawn when zoomed out.