The next 2 characters will be Blair and normal Carly Carmine. They usually do 1 new character per month, and I'm thinking Aigami might be coming up soon also, but we'll see. Duke and DSOD Bakura would also be cool.
I feel like Atticus/Nightshroud is going to be coming soon just cause of the whole Blair and Hassleberry Tag duel and Jaden/Yubel and Alexa Tag Duel. If Konami continues with the pattern they have been with GX the next character event could be Truman or Makoto and then after that Atticus as that's when the whole event with Nightshroud truly begins.