Quote Originally Posted by Jean-Marie View Post
This has been answered already. Madara could have been lying when stating that Obito just sliped through to his cave. And indeed when Obito exists, the place above is the cave entrance and is not the boulders. Many do think that Zetsu went on the many battlefields and decades long to get dying shinobies. Madara didn't necessarily need Obito. Obito wasn't even that special. Maybe there was some requirement about the ability of the chosen shinobies but it doesn't looks like high requirements. Madara was planning to boost him with extra powers anyway. Probably more requierements about the personalia: someone who could be manipulated and who could remain trustful and commited after Madara's death. Obito might not even be the first shinobi that Madara tried to use. I have really no issue with the sudden appearence of Obito in Madara's cave.
The suicide od old Madara is more mysterious to me as he did it at a time it seemed there were still too much jeopardy in the plan. But even for this: we might learn later from the story that the plan was foolproof.
Let's re-examine what happend.

1) Madara steals Hashirama's cells, and infuses himself with the cells, hoping that he'd gain Hashirama's Senju power and with it Evolve his eyes into Rinnegans.

2) The Plan did not immediately work, but years later after Madara had grown old and near death the Senju Cells Kicked in and Madara's Sharingans finally did evolve into Rinnegans.

3) Madara Gave his Rinnegans to Nagato, a gifted Uzumaki who had the potential to control the Geddou Mazou. In the hopes the Nagato would one day grow up and use the Rinne Tensei to Bring Madara back to life

Now we are not told how Madara planned to ensure that Nagatao would bring him back.
We know that Nagato himself had no Knowledge of Madara.

However Madara planned to use a Proxy or an Avatar to guide Nagato to bring Madara back.
Spoiler!

The Proxy Must be able to control the Geddou Mazou, and must be able to use Mokuton.
Spoiler!

The proxy Is PREFERABLY an Uchiha, since only an Uchiha could use the Techniques Madara taught/gave Obito

So Madara had to find a Proxy who was willing to go along with his plan, was able to use Mokuton/control Geddou Mazou, and who was able skillfully guide and use Nagato to bring Madara back from the dead.

Obito was the right Candidate because he could be persuaded to give up on Life/The world as it is, and accept Madara's Vision. He lost Half his body and thus had a massive amount of Hashirama's cells in him. As an Uchiha, he could pretend to be madara and he could use the Uchiha Kinjutsu and Inyouton.

Madara must be extremely Lucky that Obito came along, at the least.