I'm currently re-reading The Lord of the Rings for the umpteenth time, and a question occurred to me.
When Gandalf hears from Faramir that Frodo and Sam were heading for Cirith Ungol, he is shocked. Obviously, he hadn't planned to take them that way if he had survived Moria. So, what was his plan? We know he couldn't have got into Mordor via the Morannon. Is there any clue anywhere of the route he had been planning to take to get to Mount Doom?
As others have already answered, there was no distinct plan other than to send the ring off towards Mordor in hopes of destroying it. Elrond specifically calls this out and suggests that the lack of a plan is a virtue in and of itself:
I think that this task is appointed for you, Frodo; and that if you do not find a way, no one will. This is the hour of the Shire-folk, when they arise from their quiet fields to shake the towers and counsels of the Great. Who of all the Wise could have foreseen it? Or, if they are wise, why should they expect to know it, until the hour has struck?
Before the fellowship departs he cautions:
Look not too far ahead!
The Plan, espoused by the Wise is to not hold too dearly to any one course of action. A loosely formulated plan allows the hand of Providence to play a role in its success. Boromir provides an example of a specific plan, influenced by ideology and carried out by force of will, that goes astray. Boromir's singular goal is to bring the ring to Gondor, unable to deviate from this plan he uses coercion and force to further his plan and ultimately is tempted by the Ring to attack Frodo.
Evil, in The Lord of the Rings is often characterized as unyielding uniformity, a Plan; while Good is aligned with natural and organic processes that deal with change rather than trying to control it. So Gandalf's plan allows events to unfold naturally without the domination of his, or anyone else's will.
Aragorn was of the opinion that Gandalf didn't have a specific plan beyond Lórien - he was intending to talk to Galadriel and possibly take a gander in her mirror, and see if he could cook up a plan on the way.
That same page has a couple of other theories but none seem to stem from the source material.
To clarify, the statement above is based on first page of Chapter 8 of Book Two (hat tip Kate Ebneter):
"We have not decided our course," said Aragorn. "Beyond Lothlórien I do not know what Gandalf intended to do. Indeed I do not think that even he had any clear purpose."
I have always considered - largely based on the quote from Aragorn - that Gandalf had no ideas and no plans beyond Lothlórien. I think the evidence suggests that he rarely had any clear and decisive plans about how to do things, even though he had clear plans about what had to be achieved. He had very clear plan to take the ring to Mordor and destroy it in the Cracks of Doom, but exactly how this was going to be achieved in practice, I doubt he really knew.
I suspect that he may have had some idea not unlike the one he finally carried out, to take a force up to the main gate, distract Sauron, and hope the hobbits could sneak in. But I am sure it was not clearly thought out. He strikes me as more a big picture person that a details person.
It seems to me that Frodo and Sam could have easily crossed over into Mordor from South Ithilien via the pass at the source of the River Poros. OTOH, they would need to have crossed at least double the distance through the deserts of Mordor. But this would not have been insurmountable since the distance from Cirith Ungol was crossed with only one skin of water with only a couple of refills. If they hadn't gone up the Morgul Vale, they would have had 2 water skins available.
Gandalf does indeed have a plan for getting into Mordor, he just isn't consciously aware that that's what his plan is for. Gandalf's plan, as expressed to Frodo in the 'many that die deserve life' speech in Moria, is this: keep Gollum alive. Gandalf knows, or at least strongly suspects, that this will achieve something ('some part to play yet, for good or ill, before the end'), although he doesn't know that one of the particular things it will achieve is finding a route into Mordor.
I will try to keep my answer very short.
Firstly, this is an epic story and fictional, the characters play the writers song. So yea, Gandalf’s plans are unknown because Tolkien, as far as I know, never put down all his alternative ideas for the Lord of the Rings, like the plan for the Community of the ring. That will be kept a mystery forever unless his son, Christopher discovers some papers in which appear some possible ideas for the Community’s roads.
Secondly, yea if I were Gandalf, I would have also planned something like the eagle plan. I would have asked Radagast, when Saruman uses him to lure Gandalf into Orthanc, to send some friends “eagles” to go around spying the Enemy and also to get them ready and prepared to come into Lórien, when the time comes, or having at least one eagle messenger (Gwaihir) flying between Lórien and the eagles nests. If you remember after defeating the Balrog at the top of Zirakzigil Gandalf arrives into Lórien with Gwaihir’s help.
So yeah the eagle plan theory basics are totally possible and are correct for me. But don’t use the wrong ideas from the films “butterfly”, like Gandalf casting the eagle spell so freely and cheaply.
And lastly, we should already know that the eagles always appear in key points around Gandalf, when he is in total peril and nothing else could have been written that would have either way broken the logic of the Third Age marc. I don’t want to see for example a Valar saving the day, although the eagles are like sons of Manwë.
Tolkien put the eagles and their home on the Misty Mountains, that’s all.
"Far up the northern wall of the Morgul Pass, far above the main roadway, was a steep secondary pass reached by a perilous stairway, and known as Cirith Ungol." http://en.wikipedia.org/wiki/Minor_places_in_Middle-earth#M
– tonylo Oct 15 '11 at 18:03