[Tuor of Gondolin]

I tend to agree with Glaurung about
Gandalf's returning. Had he not done so it
would have been (as far as surprise and plot
alteration) rather like Janet Leigh in Psycho .
And, like Bilbo in The Hobbit, it would have put
the others in a situation where they'd have
had to, as it were, "grow up" sooner. While I
like the Gandalf and the Three Walkers meeting
in TTT Gandalf in the later story is less interesting
and it wouldn't be too hard to alter the story
(such as the relief of Helm's Deep) to compensate
for his absence.