In The Hobbit, Thorin sends Bilbo in first to scout out the mountain and maybe snatch the Arkenstone if convenient, all ideally without waking the dragon (Smaug).
Of course, the dragon is awakened and Bard ends up killing it.
What was Thorin's plan should Bilbo have succeeded in his task without waking the dragon? Naff off with it and call it a day? Use it to usurp/rally Dáin and lead the Dwarves of the Iron Hills in a full-frontal assault against Smaug?
Depending on the time you look at Thorin may or may not have had much of a plan. In the early meetings with Gandalf, Thorin suggests storming the mountain by force, routing the Dragon and reclaiming the Arkenstone:
I [Gandalf] promised to help him if I could. I was as eager as he was to see the end of Smaug, but Thorin was all for plans of battle and war, as if he were really King Thorin the Second, and I could see no hope in that.
Unfinished Tales, Part III: The Third Age, III: The Quest of Erebor
And in the novel itself:
"[Storming the front gate] would be no good," said the wizard, "not without a mighty Warrior, even a Hero. I tried to find one; but warriors are busy fighting one another in distant lands, and in this neighbourhood heroes are scarce, or simply not to be found. Swords in these parts are mostly blunt, and axes are used for trees, and shields as cradles or dish-covers; and dragons are comfortably far-off (and therefore legendary). That is why I settled on burglary.
The Hobbit, Chapter 1: An Unexpected Journey
As the novel itself draws on, it seems more likely that Thorin doesn't have much of a plan and just seems to be following Gandalf's suggestion.
So I rode off back to Thorin in haste, to tackle the difficult task of persuading him to put aside his lofty designs and go secretly - and take Bilbo with him.
But that was not enough for me. I knew in my heart that Bilbo must go with him, or the whole quest would be a failure - or, as I should say now, the far more important events by the way would not come to pass. So I had still to persuade Thorin to take him.... Though I argued with him far into the night after Bilbo had retired, it was not finally settled until early the next morning. "Thorin was contemptuous and suspicious. 'He is soft,' he snorted.
... 'If this hobbit goes with you, you will succeed. If not, you will fail. A foresight is on me, and I am warning you.'
'... if you flout my advice, you will walk to disaster.'
...So it was that the Quest of Erebor set out. I do not suppose that when it started Thorin had any real hope of destroying Smaug. There was no hope. Yet it happened. But alas!
Unfinished Tales, Part III: The Third Age, III: The Quest of Erebor
The above is a heavily condensed version of the excerpt in the Unfinished Tales. But it seems to show that Gandalf was convinced there was not much hope in war, or the destruction of Smaug. Just that Bilbo had to be taken along and that without him it would be destined to fail. Whether this was because he would be able to claim the Arkenstone and hearken the hearts of the Dwarves, or for any other purpose is anybody’s guess.
Thorin had no realistic plan at the outset but that didn't seem to matter to him. He was the rightful King Under The Mountain and he was going to reclaim his kingdom or die trying. Warriors do not fear death, and kings like Thorin cannot bear to live as vagabonds exiled from their kingdoms.
He started out with what was familiar. He knew how to wage war, and he had loyal subjects who would call him King and would spend their lives to restore the glory of Erebor. But this idea was not really a plan, and it vastly underestimated the enemy.
Notice how in chapter 1 of The Hobbit there was really no discussion of how Smaug would be killed or expelled. They gathered, ate, and sang songs about the great kingdom that was lost and would be regained. A motivational session. That was it. Bilbo, and we the readers, are left to assume that they know what to do and it will be as easy as they seem to believe.
Gandalf provided all the actual early planning, and that wasn't much. Gandalf knew there was no chance that even thirteen great dwarvish warriors could just march into the mountain and kill or evict the dragon. Gandalf somehow knew that a burglar was necessary on this adventure, and that Bilbo of all people must be that burglar. Gandalf persuaded Thorin to seek refuge in Rivendell. Without Elrond, the means to enter through the side gate would have remained unknown.
Through their brushes with death at the hands of the trolls, the goblins, the wargs, and helpful encounters with Elrond, the Eagles, and Beorn, Thorin and the entire party came to rely more and more on Gandalf for guidance, assistance and survival. Notice how upset they were with Gandalf when he suddenly rode off without them at the edge of Mirkwood! They kept going, but this revealed just how dependent they were on Gandalf by that point. Thorin's plan by that point basically was:
When they actually arrived at the Mountain and Gandalf still was not with them, Thorin didn't want to admit that after all this he had no real plan. So he ... sent in the burglar. "Go burgle something!" After all, Gandalf insisted this hobbit was essential to the company, right?
Finally, after the dragon was aroused, and they all realized exactly how outclassed they were, all they could do was huddle inside the secret mountainside entrance and wait for something to happen. Thorin with them. Bard finally took care of business and led the people of Dale to start the disaster recovery effort brought on by the recklessness of Thorin Oakenshield, who now had to either pay reparation or be taught a lesson.
Thorin had no realistic plan for defeating the dragon at any point. All he had was dwarvish stubbornness, kingly valor, a Company, a Wizard, and a Burglar. And that was all he needed.
There was a lot of enthusiasm but no real plan at that point. To make a plan, you need to scout the situation, create a few alternatives and pick up the best. Then follow the plan.
They failed at step 0 of plan making: gather the intel before making plans.
Step 0: Gather intel before making plans.
Step 1: Make plans A, B and C
Step 2: Choose the best alternative
Step 3: Act according to chosen plan
Or failed step 2, if you go by this list.
Step 1: Go to location. (90% of the book is about this. Successfully completed)
Step 2: Scout and assess the situation. (Failed)
Step 3: Make a few plans how to get to treasure with current data you have (edit: and get rid of the dragon if it is still alive in the process).
Step 4: Choose the best plan of the bunch.
Step 5: Implement the plan.
They succeeded at step 1, they failed at step 2. Steps 3, 4 and 5 didn't happen because they were dependent on step 2.
