KO2: Get rid of the most of the grinding, implement comboing (fast-fingeredness/timing) as natural part of the game rather than being bug, graphical updates (don't make several items and skills look identical). Make the game P2P.
There are also probably several good ideas that could be stolen from other games.
Right now, there is so much focus on the grinding. It should not be taking as many hours to get to max level (not only 80, but also 83) as it does today. Instead, there should be a heavy focus on endgame content. CZ should be gotten rid off, or given a different role. There should be several different PvP zones with different purposes and different dynamics; e.g. more stuff like ED, only less event-oriented. The lunar wars could probably be removed, or revamped completely (less 24123 people running at the same place getting bombed by mages).
Events (apart from lunar wars, in case they still exist) should not start at a certain time, instead you should wait for enough people to sign up for them. E.g. you can do three BDWs per day, but you can do them in a row if you like; and at any time of day. Alternatively, still make a limit on how many times a character can attend an event per day (if desired), but have sign-ups every 30 minutes/every hour, or something similar.
That's a start.