This is exactly the problem right here. Konami can't think outside the box more to make unlock events that don't have to fit in with the anime plot timeline. They should just make him unlockable through a bingo mission if it is that hard to come up with a new unlock event with dialogue.
They were clearly just making a joke on the fact he went missing in the GX universe for quite a while...
I feel like Atticus is still one of the most likely remaining GX characters either way, especially since they hinted at Darkness/Nightshroud being a thing, since he had a pretty strong connection to that plot.
However, if they add everything too soon, some duel worlds will run out of characters which is not good. Delaying him would make everything more interesting, I mean... they already gave us Jaden/Yubel I guess we can't really complain.
They're never going to run out of characters. DM has like 10 billion characters already, and I could easily name another half a dozen that would make sense without delving into dumb picks like Zygore or Seeker. It's mostly down to pacing. Obviously if you release a bunch of characters from the same universe at once, people get burned out by it.