I've given up hope of seeing any more Lit-verse characters on the covers. At this point they seem to be pretty much sticking with the canon characters. I can understand that though, it's got to be a lot cheaper and easier to just use preexisting photos than to hire a model for a new character.
The other consideration is that casual browsers would tend to say (and I've heard them) "Who the hell is that supposed to be?"
It's not too bad if the new lit character has something distinctively alien to their look (ie, "Oh look, it's an Andorian!"
for Shar), or is a recognisable past guest star, but for random human, non canonical crewmembers it just doesn't work. Especially if readers have already decided a particular look in the minds from previous novels.
We've had debates here about who two
of the characters are meant to be on the cover of "TNG: Reunion", and people have asked for character lists for the four-part covers of "DS9: Mission Gamma". There are people who don't actually recognise that mustachioed, long-haired Sulu
is on the cover of "The Entropy Effect".
I'm sure there is past correlation on sales/covers on record at Simon & Schuster. Covers with characters not readily recogisable probably don't sell as well, just as we know that covers featuring Spock and Picard do.