More of a tutorial episode than a build perhaps but how about a show based around the DMX512 lighting protocol ?
Ideally looking at the protocol, its timing quirks when attempting to use the likes of a USART, various microcontroller options, RS485 line drivers/receivers and then a worked example of transmitting DMX512 and receiving DMX512 in such a way to allow it to be easily incorporated into other projects.
As a worked example then perhaps a couple of diagnostics tools, one to send a DMX512 test signal and one to decode it which can then be used back-to-back or to test real world fixtures/controllers.