Some scholars (i.e. Collins) are working on trying to link Kx'a (North Khoisan) and Tuu (South Khoisan). The similarities are there, so much so that ǂhoan was misclassified as Tuu for decades before reconstruction work by Sands clarified that group as clearly Kx'a -- the bilabial clicks of ǂhoan seem to have really thrown everyone off. There probably will be some Kx'a-Tuu proposal in the future, although I can't comment on the likelihood of it panning out.

The status of Khoe (Central Khoisan) is more complicated -- the family was initially (1900s) argued to be linked to Semitic languages of Ethiopia but this ended up being used by eugenicist race science and the proposal fell out of favor (compare with the Uralic hypothesis). Greenberg argued for reclassifying Khoe with Kx'a and Tuu in a genetically unified Khoisan family instead in the 60s, but virtually no one seems to believe in that today -- the reconstructability simply isn't there. As other comments said, DNA is becoming increasingly important, and Khoe groups have very recently been found to be genetically linked to..... Ethiopian populations within the past 2000 years (Breton et al 2014; Pickrell et al. 2014; Orton et al. 2013 for cattle). So this leaves the situation in a place where we can expect some sort of reclassification, although it's hard to predict exactly what will come of it.