About how the series will end, he confessed that until recently, he still wanted to "betray" the fans with a dark ending (the famous anecdote about a conclusion similar to that of the film adaptation of Stephen King's The Mist where everybody dies), but as of yet he is unsure about the path he will follow, so essentially, anything could happen —I personally don't believe him in this part, and I think he indeed knows what he will do, but for obvious reasons doesn't want to tell.
What follows is a direct translation of this part of the interview courtesy of Fukushuu:
Mr. Horiguchi shared with us his expectations for the future, so now, Isayama-san - please let us ask you what your mind has planned for the future.
Isayama: Tankobon Volume 20 will be released on August 9th, and it will approach the climax of the story. So everyone: please keep reading until the epilogue.
Horiguchi: So we will know the truth behind SnK’s world? I really want to know the answer. (Laughs)
Isayama: Since I don’t want to expand the story any further than my current plan, I will hurry up and reveal the answer soon!
Horiguchi: No no no, wait a second, I didn’t mean to push you. (Laughs) I still want to enjoy the manga for a while longer. That being said, my own wish is that there can be a happy ending where people can live freely with no walls. To me a sad and dark ending is, well…
Isayama: Until recently, I used to fully take the stance of creating a dark ending. Before, I wished I could shock those who have been following the manga. And all I thought about was to somehow betray the readers who are enthusiastic about the work. But now, as I am turning a little bit older, my mindset has shifted. I am still not sure how the manga will eventually turn out.
“What decisions will Eren and his friends make?” We are looking forward to figuring it out along with Isayama-san’s changes.
Isayama: Even though I’ve already designed the upcoming plot, I will still keep all possibilities open and see how the characters react by themselves when I draw the draft organically. That means nothing has been settled down yet.