The Original Plan for the Fellowship?
 

So obviously Gandalf falling in Moria and Frodo fleeing after Boromir's attack were not planned. There were a lot of hasty improvisation going on there.

But Gandalf and Aragorn are no fools, they wouldn't have set out without an "endgame." Is there any indication of what they planned to do had the Fellowship remained together? How did they plan to get into Mordor or to distract Sauron?

If there is no canon on this, do the posters here have any theories?

It appears that there were different stages and they would improvise as needed. The first was the trip up to Hollin. The second was getting over the mountain which they were prevented from doing. They discussed the alternatives before embarking.

If Gandalf had any ultimate ideas for the direction of the Fellowship after Lórien he did not share them with Aragorn.

FOTR The Breaking of the Fellowship

It looks like Gandalf really didn't have a definite plan for getting into Mordor, though it seems he would not have wanted the Pass of Cirith Ungol, based upon his reaction to Faramir's news that Frodo was heading there. Gandalf may have had some foresight that somehow Frodo would infiltrate Mordor, but not how he would accomplish it. Gandalf probably would have improvised some plan along the line of what Aragorn proposed, going with Frodo and leaving Aragorn to go with Boromir to Minas Tirith. How that would have played out is impossible to guess, though it's possible that Gandalf's presence would have been detected somehow by the Nazgûl or by Sauron himself, and that could have been disastrous.

Perhaps though I think he could conceal himself. He did infiltrate Dol Guldur.

Well, Sauron was in a sort of powered-down state then, and I don't think security was a very high priority in Dol Guldur. By the time of the War of the Ring it was a different story.

I think they all had faith. Strong faith. Valar had "plan", and probably Gandalf was aware of it to some degree. As for the start Fellowship remaining together, no was bound to the quest. No catastrophe was going to take place. And I cannot not think Gandalf was planning to enter Mordor. As Inzil said he could be spotted by Sauron and his minions. I don't think Gandalf was going to risk that(it would have turned out to be Sauron's victory). According to Gandalf the whole quest of destruction of the Ring thing was a 'fool's hope'. They all believed in "[doing] something is better than [doing] nothing." So they tried to do their best having faith in the higher power. Also, things "by chance" turned out to be right. It was all about a plan but of Higher Authority. :3

I think Gandalf had no fixed plan on purpose. If you make a plan from the outset, you tend to try to stick to it even though circumstances change, and the result might not be the best. So some bits, like Moria (and, had Gandalf lived to see the day, Parth Galen, and Henneth Annun, and the Crossings, and etc), had to be decided on the spot.

Gandalf made sure he knew his options. That's what the scouts from Rivendell were for. He knew which ways were open and which were not.

Some parts, like the getting-into-Mordor part, he had no solution for, so he could not have made a plan at that time, or really at any time. He probably thought to worry about those mountains when he got to them. He would have to think of it on the spot. And sometimes it's not that bad of an idea - once you see something firsthand you can try things and do things and see potential solutions, something you can't accomplish from a distance.

Mostly, though, I think his goal was to keep the "plan" as flexible as it could be. Like none of the Fellowship bound themselves to the Quest with oaths, Gandalf did not bind the Quest to a plan, since no one knows where it would lead.

Hard to say, because as quoted not even Aragorn was sure what the plan would be. Personally I think that the Fellowship would have eventually have split up. Aragorn and Boromir, possibly with the other Hobbits would have gone to Minas Tirith. Sam and Frodo would have gone together most likely with Gandalf, but it's hard to know. Legolas and Gimli are complete wildcards and could fit in with either party. I don't think Gandalf would want to bring the ring to where Denethor was, more so when he brought the Heir of Isildur with it.

I think it was still a grievous place to try to not only infiltrate but to escape from. When Boromir spoke of how entering Moria would be like walking up to the gates of Mordor, Gandalf told him that is was hard to enter and then leave Sauron's domains, but he would not lead them into Moria if there was no way out.

Gandalf was saying it was not like entering one of Sauron's strongholds, even his lesser one, if they were to go under the mountain through Moria.

Aragorn's endgame was to head to Gondor with Boromir because he believed that the dreams Faramir and Boromir had were a summons for him. However, he believed Gandalf probably had no concrete plans past Lothlórien.

As for Aragorn's part in now bearing Gandalf's burden as leader of the Company he would follow the Ring, "yet what help could he or any of the Company give to Frodo, save to walk blindly with him into darkness?" [p. 435] So I'd say Aragorn had no plan on how to get into Mordor or to distract Sauron. Though Boromir did suggest they should go with force into Mordor rather than sneak in as they were, but not necessarily to destroy the Ring.

I think Aragorn says somewhere he had tentatively planned to go with Frodo and Sam (and Gimli) with Boromir taking Merry and Pippin to Minas Tirith. Not sure about the skate boarding elf. It makes sense to take a dwarf to help getting through the borders (probably by some underground passage---did Aragorn know about Shelob and Co.?).

Aragorn had interviewed Gollum (or been present when Gandalf did so) and knew that Smeagol had gone in and come out. I doubt Gollum actually said how or gave directions, but the Ranger probably reckoned that if there was a way he could find it.

Moreover, Aragorn had spent time in Gondor, and it's clear from Faramir's comments that something at least was known of Cirith Ungol. Now, whether Aragorn would have dared bring the Ring so close to Minas Morgul is another question.

When Frodo went off to think about what he would do they were discussing themselves what to do. The Hobbits were debating whether they should convince Frodo not to go to Mordor if he wanted to go. Aragorn told them that it was the Ring bearer's choice. If Frodo did choose to go, Aragorn said he and Sam would go, and Gimli said he'd go too, and that Merry and Pippin go with Boromir to Gondor with Legolas unless he too chose to go with Frodo.