Sir Ian McKellen let it be known during a fan event that he along with Elijah Wood are
returning as Gandalf and Frodo for The Lord of the Rings: The Hunt for Gollum. The movie will directed by Andy Serkis who will also play Gollum. The current goal is a December 17, 2027 release date which puts the movie against Avengers: Secret Wars so likely one of them is moving.
“I hear there’s going to be another movie set in Middle-earth, and it’s
going to start filming in May. It’s going to be directed by Gollum, and
it’s all about Gollum,” he told the For The Love Of Fantasy event, per a
video.
“I’ll tell you two secrets about the casting: There’s a character in the
movie called Frodo, and there’s a character in the movie called
Gandalf,” he continued, knowingly adding: “Apart from that, my lips are
sealed.”
Exactly how much they are in the film remains to be seen. In the Fellowship of the Ring movie, there the early scene where Gandalf confronts Bilbo about the ring, suspicious of what it actually was. In what we now know was tremendous will power, Bilbo leaves the ring behind to go on another adventure. Meanwhile Frodo is told he inherited Bags End, told to keep the ring safe and Gandalf goes off to find more information. This is followed by a moment when hear Gollum speak of the "Shire!" and "Baggins" sending the Ringwraiths towards Hobbiton. Then there is a scene of Gandalf at Minas Tirith finding enough information to confirm his suspicions on the ring. The film then quickly cuts to Gandalf returning to Bags End and using fire to verify what it is.
How much time passed between Gandalf leaving ring in Frodo's care and then coming back to verify it with Frodo? The film does not tell you. Could have been days, could have been years. The novel tells you - 19 years. Then there is the time between Gollum losing the ring (seen in The Hobbit films) and escaping the Ringwraiths to track the ring down himself. That was 70 years.
Material wise there really isn't that much information in the novels about what Gollum did during those 70 years of searching besides get captured a few times, once by Aragorn as part of Gandalf's information hunt. Frodo's part in events during that time is unknown but its assumed chilling at home while Gandalf was mostly traveling all over Middle-Earth collecting nuggets of information about the ring. All this is to say, it doesn't seem likely the two characters will have large roles in the film as their part in the hunt was either non-existent (Frodo) or guy getting updates while doing other things (Gandalf). Aragorn could have a large part depending on how the story is told. Much like the trilogy, the start of the Fellowship to defeat of Sauron was around 13 month odyssey according to the books, time will be compressed to make it better fit continuity of the films.
