Types colormap I've sometimes wanted to use, but which never seems to be included in packages or evaluations of colormaps, are 2-dimensional and cyclic colormaps.
I often would like to visualize two related values in the same plot, so a 2-dimensional map would often be helpful. Cyclic maps come up in the context of visulaizing complex functions, but also in other contexts. There are some premade maps for that (e.g. Mathematica's BalancedHue), but they are much rarer and I have never seen a discussion about colornaps which includes them.
