We did get into World War 1, so going into the Korean War is a good possibility. I wouldn't go any farther than that though.
The reason the WW1 arena is "dead" is there isn't a goal to be had there, with the exception of furballing. Toss in bombers, things to destroy and capture and various other things, then you would probably see more people in it. I would guess that it would be like Mid War in terms of numbers though. The Korean War Arena would likely have more numbers than Mid War on a daily bases unless you made it like current WW1 arena. Then it would have the same, if not more, people in it than the WW1 arena, but not enough to get out of the label of being "dead".