You can ask NESdev question on Stack Overflow, however you'll more likely to have good answer (*) by asking on Retro-computing stack exchange, or even electronics stack exchange if your question is pure hardware.
If you would like to create a NES-dev only stack exchange you can try to submit a proposal in Area-51, however I doubt it makes sense since Retro-computing stack exchange is already here.
I think both formats are good for different purpose, Stack Exchange is so successful because it's just questions and answer, and the discussion is purposedly prohibited, to avoid the "problem" of threads derailing off-topic. Now, forums are still great in that we can give more details and derail off-topic which means having more interesting discussions, more like in real life; while stack overflow is more like if you were in school and asked a question to your teacher.
There's currently 27 questions tagged NES on Retro-computing stack exchange.(*) Personally I've had rather poor example with the original Stack Overflow ; The mentality there is 1) you suck, 2) your question suck, 3) but since you're so terrible at it we'll solve that problem for you (and down vote you). Other SE network pages doesn't have this nearly as much. Or maybe it's just my imagination.