In an interview with The Daily Beast, titled “‘No Regrets’: Peter Jackson Says Goodbye to Middle-Earth“, Jackson talks about the difficulties in returning to Middle-earth as well as his decisions to include Legolas and Tauriel.
Talking about Tauriel, Peter Jackson admits that the decision has been controversial but he stands by it:
Yeah, there’s negative reaction from Tolkien fans, but then you have a nine-year-old girl who goes to the movie and she’s delighted that there’s a character she can relate to. So it depends on what side of the track you want to come from. […] Then you’ve got the king and you’ve got a son, and stories are best told with three people, not two, because then you can create conflicts and triangles. So we wanted a third elf character. Was this a chance to put a female role in the story? Because there are so few female roles. Also you do have a lot of young girls seeing this film, and they should have somebody in there who they can empathize with. It was a very cold-blooded decision. Yes, OK, a female elf. And that was how it came about…
He also talks about the possibility of filming The Silmarillion:
The truth of it is, they probably will one day [be made]. The Tolkien estate doesn’t want to release the films because it’s a decision that the current executors of the estate believe in. But at some point that may change, the executors may change, and those rights may suddenly surface. […] But if it was something connected to this mythology that we’ve done, if I had the energy and the strength then I would really want to do it. Never say never!