Personally,
I want to take some of the forensics stuff I’ve done with microcontrollers, then BB-xM, then BB, then BBB, and port it to the X15 for the USB 3.0 support. To me that USB 3.0 support is huge as no other small computer boards have this. Imaging 1 TB drives at USB 2.0 speeds is a serious downer.
It's funny, in the lab with an alpha x15's i don't use sata anymore,
instead i'm just maxing out a usb 3.0 flash drive. 
Regards,
With the 4 PRUs and all the interest in driving LEDs with PRUs, one
thought I had was doing an HD "jumbotron". With 1080p60 decode and 157
GPIOs, I think it is interesting. I did some back-of-the-napkin
calculations on it and had some concerns that I've forgotten now, but
I don't think they were such that it was impossible, especially if
coupled with networked BeagleBone Blacks if additional drive reach was
required. One item that is cool is you could still add cameras and
vision processing on top of it.
Vision and machine control apps should also be really nice for X15.
I'd love to use it for our hard real-time neural and cardiac electrophysiology work. Lots of users in the academic space would love such a system. our current users of the BBB-based system (puggleboard) are pretty happy with it, too.