The former Lost writers explained that the May 13 episode of the fairytale series represents "a step into something new".
"How the audience perceives it, we can't anticipate, but for us it does change the playing field," Horowitz told TV Line.
"We like to think what we're doing is evolving the show so that it remains true to what it's been this year, but it takes a step forward into something new.
"And one of our other goals with the finale is that at the end of it you say, 'What the hell are they going to do next?'"
Kitsis added that the Once Upon a Time finale will "tie up loose ends" of existing characters rather than introduce new ones.
"It will introduce some new story ideas," he said. "But as far as new characters, if you're talking, like, Michelle Rodriguez showing up at the end of a Lost finale, no.
"That's not to say there won't be new characters next year; but this finale is about the characters we've introduced."
Once Upon a Time concludes Sunday (May 13) at 8/7c on ABC. In the UK it airs Sundays on Channel 5.