I've been doing a lot of senior photo shoots, and also some general portrait work over the last few years and I was thinking that a prime lens might be in order for better DOF and results....
When you say better DOF what do you actually mean? A shallower DOF or deeper DOF or a particular bokeh look? Your 70-200 is a fantastic portrait lens, it's the go-to lens for portrait shooters across the planet. I'd suggest looking at the exif data on your portrait shots and see what focal length is your most used for what you consider the best results. PS Bridge or Lightroom will sort these for you. That might help aim you towards a well informed decision for a 50, an 85 or a 135 or just to stay with what you've already got.
Looking at your current lens list, I see a gap at the wider end if you're looking for quality. You'll pick up a good pre-owned 24-105 for the $amount in your budget.
I'd like to have a shallower DOF. I know I can't get much with Crop, but using a faster lens should help, correct?
Also as I mentioned, I'll be leaving the crop world and heading towards FF around the end of the year, as I save up for the 5D3. Once I go FF, I will no longer have the 15-85, as it's for Crop bodies only. If I can collect enough, I could get the 5D3 with the 24-105 as a kit. Until than, I'd like to have a little shallower DOF in my photos.