Same here. The ZX manual that came with a list of Z80 opcodes was how I learnt assembly/machine code programming. I lived in a real backwaters, and getting the ZX Spectrum as a gift from a distant uncle changed my life.
Had no other books, no community, absolutely no other resources to learn anything. But the ZX and its manuals got me to where I could write machine code programs without even looking up the opcodes.
When I visited the Centre for Computing History (http://www.computinghistory.org.uk/) I noticed that as well as having games for their collection of 1980s home computers, they also had developer tools. Having read about Ian Bogost's "A Slow Year" (for the Atari 2600) it occurred to me that making new games for old machines using "period instruments" might be an interesting exercise. What I had in mind was games that would never have been written at the time, but that would have been technically feasible.
That's as far as the idea got because, er, I'm not a games programmer. As close as I got to an actual idea was that perhaps it would be possible to write a Tetris clone for the ZX Spectrum. Can anybody think of something better?
