Actually, I agree. Try to stay for a couple of days there if you can. I am not sure if I would give up Tokyo for Hong Kong in this case, not because I liked it better, but because it's very different from the other three places in my opinion. But that's a tough call and I would consider it, especially because I enjoyed HKG more than TYO and it's a helluva lot cheaper. Especially right now. Maybe substitute Hong Kong for New York since flights to NY are cheap and short. I sure miss traveling as much.

What you might also consider is breaking the trip into two parts. That would give you a bit of extra time. I'll use my itinerary as an example:

CPH-LON-NYC-SFO-HKG-TYO-HKG-DPS-HKG-BKK-LON
arrange your own round trip from london to copenhagen and back
LON-CAI-LON-CPH

That way you can spend a few extra days on the long trip (thereby adding hong kong) and do the short one when you can get more time off. At first glance, lon-cph flights don't look so cheap, but maybe use FF miles or something. Would also give you a degree of flexibility in most cases.