I think it would be very useful to get back the functionality of the original Nik suite: which means to be able to use these tools in Greyscale and CMYK mode. The latter is extremely important for sharpening! When converting to greyscale I sometimes go back to Silver Efex for adjusting some elemnts using U-points.
to work in rgb-mode is fine but to have an info-window like in photoshop, where you can see the cmyk-percentage, would be very helpful.