Really great ideas! I for one think these could be extremely useful! Like for example, right now I just wasted a lot of time creating a template for myself that has golden spiral and rule of thirds as layers because I want to rely to those heavily in future.

But, to probably set things in motion, I propose that the most important/useful/vital ideas are gathered and implementation started already, and more situational ones can be implemented later. This was at least some of them can make it into 2.9 in a hopefully not too distant future.

Hereby I suggest four things to be implemented as soon as possible:
[list=1][*]Assistant layers: Important for organizing existing and future assistants, and easily toggle visibility for only selected ones.
[*]Simple linear guides, similar to the "Guides" in Photoshop, that can be used for easily dividing the canvas into areas, or mark centerlines etc. These are pretty vital in my opinion, and if you come from Photoshop, you can't imagine life without them.
[list=1][*]They can be created easily by dragging [from the] the rulers into the canvas area, and deleted by dragging the guide back onto the ruler.
[*]They can be snapped to, and they themselves snap to selections, canvas edges and center lines of the canvas and selection.[/list]
[*]Golden spiral and rule of thirds assistants: Very simply a vector each, that can be resized.
[*]Snapping of assistants and selections to assistants, selections, center lines and canvas edges. Snapping enabled/disabled via shortcut or from Edit menu. Modifier key makes brush snap, rather than having to enable "assistant" option in tool settings.[/list]

Sorry for the bad formatting above, forgot to delete BB code...

Also, created a wish ticket for a related idea: The ability to load vector images as assistants: https://bugs.kde.org/show_bug.cgi?id=343067

WISHGROUP: Stretchgoal