The actress - who can currently be seen in Madonna's new film W.E. - plays sinister witch Morgana in the BBC One fantasy drama.
Asked by Digital Spy if her character could be darker, she replied: "I hope so! God yeah!"
McGrath also admitted that she is still shocked by the level of fan support for Merlin.
"The show's grown up [this year] and it's moved on," she suggested. "People seem to love it. I still can't quite believe it's beloved as much as it is."
Merlin continues this Saturday at 8.10pm on BBC One.
> Merlin Katie McGrath interview: 'Fans should expect the unexpected'
Additional reporting by Morgan Jeffery
Watch Katie McGrath discuss Merlin with Digital Spy below: