It would be nice if additional colours would be supported, à la Hexachrome by Pantone (this was a 6-colour ink system which covered more than half of the PANTONE spot ink colour space and made quite vivid photo reproduction possible). Even better would be a mechanism like to Cerilica's Truism which would allow one to use arbitrary filaments and preview how they will blend when printed.
Photo reproduction should be the target here... And it looks like the software support needs work too - the obvious way to do it is being done able to import a jpeg or png to project or wrap onto the surface, a bit like texture maps in video games.
There's an app called Hueforge that produces models that color mix to reproduce photos:
It's been around for years. There are databases of filaments with their TD values and color measurements to use. The blog really sells Prusa's attempts to do this with their own PLA, but there's a long history of color mixing in the community with extensive measurements of filaments that anyone interested should check out, too.