He told Bullz-Eye that the show's production team feel pressured to top the previous 13-episode run.
"That's the challenge we're currently engaged in," he confirmed. "That's why we're going off to to the States to film for the opening two-parter, and that's why Steven [Moffat] is coming up with more dark and complicated and strange things to inflict upon Rory and Amy... to complicate their relationship."
He added that future episodes would also "mine the Doctor's relationship with River [and] reveal some pretty big truths about that relationship".
Wenger also reaffirmed series star Matt Smith's claim that the monsters featured in the US-based two-part premiere would be "on par with the Weeping Angels".
"They are quite terrifying," he claimed. "I worry for the sanity of some of our fans. I worry for our own sanity! They are properly scary. They are really going to make the Weeping Angels look like a bunch of stuffy old statues."
Doctor Who returns for a special one-off episode at Christmas on BBC One, while the sixth series is to follow in the spring of 2011.