Based on this and on what was originally released in KR and what would be up next on the list my guess is Summoner and Arcana would be next, but I highly doubt we get 2 mage classes next back to back.
right after this would be Reaper and Scouter that were both dropped in KR at the same time. (If i am remembering correctly)
So if I had to guess we will get a combination of either
Summoner and Reaper (the classes both shown in the official PVP video)
or
Arcana and Scouter
Obviously, I do not mean in the same month but rather the next ones to be announced if we get another 2 month’s roadmap.