After reading a few of the IEEE Signal Processing Magazine's series on "DSP Tips and Tricks" it struck me that these are very interesting, easily digestible, and useful tools that we could use in GNU Radio. If you have access to IEEE Xplore or the Signal Processing Magazine, search for "DSP Tips and Tricks" to get a list of all of the articles. They are all about 3 pages long and geared towards practical outcomes. Many of them focus on FPGA and fix-point DSP projects in particular, but they each teach some lessons that GNU Radio could potentially benefit from.
I would love to see a series of maybe junior or senior year ECE student projects to implement these (they probably aren't capstone-level projects, though). They'd be great hands-on projects with math and programming and you would be required to demonstrate it working with real signals.