A few things were probably a bit too predictable (a starfleet officer who is given a name and some lines but was never in the TV show? I called her death after about the second time she was mentioned)
Actually, a lot of the books over the years have added new characters who weren't just there to be killed off. All of Christie Golden's VGR novels featured a recurring character named Lyssa Campbell, who wasn't killed off until Kirsten Beyer took over the series. Similarly, all the Enterprise
novels by both Dave Stern and (Andy Mangels &) Mike Martin featured a recurring night-shift watch commander named Donna O'Neill, named after a Pocket Books staffer. Then there are the security guards Meyer and Boyajian who show up repeatedly throughout Peter David's Trek fiction (in two centuries!). And much earlier, J.M. Dillard's TOS novels gave us security chief Ingrit Tomson, security guard Lisa Nguyen, and other recurring characters, with Tomson even showing up in other authors' books from time to time.
Even a lot of the one-shot novels have added supporting crewmembers just for the sake of fleshing out the crew, something that's more feasible and often more necessary in a novel-length story where you have a lot more room to go into depth.