An idea i had a while back was to create a 'mock up' game as it might look on a computer/console that it wasn't actually ever released on.
Think of it as like a fangame but with a specific style.
Say for instance, Megaman on the Commodore 64, Alex Kidd in miracle world on the Gameboy, Kirby's dreamland on the ZX Spectrum or perhaps even a Click game? Fishhead 2 on the Atari 2600? lol.
These are two mockups i made a while ago of two games as they might look on an Amstrad cpc.
I like your idea I'm a big fan of old game systems. The game I'm working on at the moment, I'm trying to make it using the limitations imposed by games for the first Nintendo. Contrary to what you'd think it's actually kind of fun to work with such limitations. Both for the challenge but also for the learning experience of getting an idea of how hard it must have been to create a large game back then.
Sometimes very early devs didn't have the foresight to create tools to speed up their work. So they would actually have to work out things on graph paper, then punch the numbers in manually bit by bit. Even for graphics and level layouts.
But generally games didn't take all that much time. 3 Months would be about the usual deadline. or if it was a port it might be about 30 days.
There is a difference between Console development and Computer development. I think generally games for home computers in the 80's were a little more amatuer.
Nintendo changed all that with their 4-5 games per year limit.