So, South Korea has a confluence of factors that support a major raid push in the area:
1. Many areas, particularly away from major hubs, are poorly filled in leading to "no route found" errors for many cultural and historic destinations.
2. Relatively few Koreans use Waze, due to the relative popularity of Naver Maps, so few local editors.
3. Waze is the only English language mapping option I know of. Naver maps is korean only. Google maps doesn't do driving directions due to some weird legal stuff in korea.

If we can make it happen, I'd suggest the following lanes of effort:
1. Laying and connecting roads. This is the single largest issue as it's what breaks navigation to many locations
2. Road name formatting. With the recent launch of text to speech, some road names read weird. For example, near me there is a road "<72>사수로, 대구 Daegu, South Korea" which reads as "less-than 72 greater-than" Other route numbers use () or {}. This should be standardized if possible. I'd suggest "([route number]) [korean text] [English transliteration/alternate name]" or for the above example, "(72) 사수로 sasuro" You will notice the city name is already in this format: "대구 Daegu" and most street signs are in this format.
3. Resolving open items. There are a fair number of map edits I come across that have been open for over a year with not so much as a comment.
4. Anything else. Adding places, filling in missing street names if known, etc.