China is a major trade partner with the US, so it seems odd to think we would ever come into direct conflict with them. Their current economy is much stronger than Cold War USSR or Nazi Germany, let alone Iran, Iraq, North Korea or Vietnam. It's only getting stronger too.
North Korea's military is probably a joke compared to Iraq's. The real danger North Korea poses is their proximity to urban areas in South Korea. They would probably be able to do a lot of civilian damage before they were stopped. For that reason no one will ever provoke North Korea. Everyone hopes they will slowly deflate and collapse. If North Korea were to start a serious conflict without provocation, I would be very surprised if China supported them.
China won't let North Korea collapse right now, however, because they enjoy importing its natural resources at low cost, and neither China nor South Korea wants to deal directly with hundreds of thousands of starving refugees.
On the other hand, Iran would have defeated Iraq during the Iran-Iraq War if the international community hadn't stepped in. They seem the most likely country to be in direct conflict with NATO countries in the near future, but would that be significantly different from the conflicts in Iraq?
If you're looking to create realistic scenarios, I think history is right now offering some pretty compelling situations with the Arab Spring. Missions inspired by these civil conflicts can offer challenges other than just dodging SAMs. The trick, I think, is figuring out just what can be done about that in the DCS mission editor on a map of Georgia. I've never made a mission before and I'm still a DCS newbie, but I've lately been tempted to try.