Why is it people think that because the 2600 is "simple" (at least in terms of what level of graphics it can produce) that it will be easy to emulate?
The reason it isn't is because the 6507 and the TIA are "tightly coupled" - each can do things which have an immediate impact on the other. The 7800 and NES are loosely coupled unless interrupts are used. But In the 2600 the 6507 & TIA could almost be regarded as a single entity.
Anyway, occasionally people suggest creating a 2600 emulator for the Propeller. In their mind they assign a cog (processor) to each function - CPU, graphics, sound, input, etc. I typically scoff at them, citing the difficulty in emulating the 2600 - if for no other reason than the clock relationship between the 6507 and the TIA.
But earlier this week I started thinking - why not give it a shot. Put together a 6502 emulator with an eye to making it usable in a 2600 project. It might be fast enough or feature complete enough, but it will be an interesting exercise to see if it's even possible.