Which international routes might save fuel and time by using a potential newly "deemed safe" North Korean air space?
I think the biggest one would be Tokyo - Beijing, and more broadly, anything in Japan north of Osaka to Beijing.



The current airway turns does a dogleg up to right above Incheon airport, (you can literally see the airport and Seoul).

You can play with http://www.gcmap.com to see great circle maps, which are a good approximation of the most efficient route.