When applying advanced color theory to painting there are several important factors we need to know and be able to manipulate about color relationships. Currently the Advanced Color Selector has several modes, some of which I personally feel are super useful for this - but you would need at least two present the same time. There are also other useful features to work with advanced color theory.

1. The hue circle with HSV square or triangle (whichever you are used to) is great for picking variations on your hue, and there are some visual methods that can be applied - for example when an object is hit by light it will change not only the value but also saturation of the color we see (highlights are less saturated) and these changes can be visually represented in the square and triangle as an imaginary U curve from which we pick colors.
2. The color disk is great to visualize how colors of light interact with each other and to create color harmony - For example, when mixing colored light, our mix result will be in between our two starter colors on the wheel - the further away the two colors are in hue (larger degree angle on the disk) the more desaturated their mix gets. So colors at 180 degree in equal measure mix to white. Also the distance of our used colors on the disk tells us how well those colors in the artwork are able to harmonize with each other. For an explanation of this approach see this: https://www.youtube.com/watch?v=4LhcNbFMkTw
3. The existing features "color history" and "colors from image" can be very useful when working with color harmony and dynamics, and could be even more useful if represented visually in relation to each other.
4. There are other useful features too, and for this reason professional Photoshop users almost always use plugins that extensively augment Photoshop's color selector, such as Coolorus (https://coolorus.com/). It would be great if Krita could sport at least some of the advanced functionality by default (and also offer the rest via free plugins).

Note: The ring and triangle picker in the popup palette could be used in tandem with the ACP set to color disk, but there are two problems: This picker is pretty small, and currently it's triangle only.

As such I propose the following features to be added to Advanced Color Picker:

1. Ability to have more than one of its modes displayed at once (Idea: Have 3 dropdown in config to select picker1, 2 and 3 that would be displayed top to bottom or left to right depending on the shape of the ACP docker, with option 2 and 3 having "none" as default.)
2. Ability to display color history or colors from image as little circles or dots on the color disk itself, similarly how the relations are demonstrated in the above video.
3. Complementary selection helpers on the hue ring, similar to Coolorus.

Additionally I propose re-introducing the ability to choose the type of picker in the popup palette, adding also the color disks.

1 is something that while it can be theoretically done by someone out there, I am not the person who would be able to do it without creating so many bugs that the selector is broken forever. I am not gonna do this, and if someone implements this I will never touch the color selector code ever again for fear of breaking it.
2. Is theoretically possible, but low on the list of things, it would be done at the same time as gamut masks would as they touch the same code.
3. Can be done already, just paste "popuppalette/usevisualcolorselector=true" somewhere near the top of your kritarc file. It is disabled because there's still bugs with the internal color selector that would need to be fixed beforehand.