I'm currently using an AVR type ATtiny44A with 12 I/O pins, to help out the main MCU and input CH/vol chips in my stereo. Even just for that, I'm running out of pins, and that's with OR'ing some signals together. Come to think of it, I could add some more hardware mods, and make my MCU program easier.
The main MCU, is 100pins or so, and seems to be failing. I don't need most of the functions on the stereo, especially all the digital and TV related inputs, so I really should start working on a total MCU replacement project, with some 64-pin AVR MCU's I have, so maybe 2 of them if I want to do it all. Hopefully the MCU lasts long enough to decode all the functions involved. But again, I can do the main ones for EXT.IN and the VFD without to many problems. For me, this is just a good learning experience.