It's industry standard to define brand colors in more than one colorspace. Usually you have at least PMS, CMYK and RGB definitions of each brand color you're deploying.
I'd love it if Illustrator (or the whole CC suite, as a matter of fact) would help this routine, like this:
Let's have a new swatch type: Cross-Colorspace Swatch. This special swatch type would enable the designer to define variations of the color in any/all color spaces, inside that one swatch definition.
Instead of defining say three different swatches, you would define one, saying: "This is Brand color 1. In cmyk colorspace it's C50M0..., in rgb colorspace it's R0G120.., it's Pantone code is PMS 20..."
This way, when you change your document's color space or paste into a document in another colorspace, for example, the Cross-Colorspace Swatch would automatically check if it has a definition ready for the target space, and override whatever default conversion rules with that definition.
For situations where it's not obvious which definition to use (for example whether you want to use the PMS or CMYK definition when your document is in CMYK mode) you would be able to force the swatch to use the version you want (say, PMS).
Further uses would include automations for creating captioned samples of the color in each colorspace, to help in creating brand books and such, for example.