These days with RFIC's coming along, interesting ways to make stable microwave oscillators, there are a couple of interesting projects for the radio amateur. Both are moving a little slowly but with help from you gentle reader they could move a lot faster:
Microwave Engineering Project adroitly led by Michelle Thompson, W5NYV and uWSDR led by several good folks. Both are worth investing some time with to see if you can help/enjoy this kind of development. Both are peopled by very strong engineers, hardware and software.