Okay, so, I've seen people asking "What happened to Nagato's other Rinnegan?", and I believe that I may have uncovered the answer.
What happened to the other Rinnegan is that Madara is keeping it in safe-keeping. For who? Himself. But why not just implant it right away? He can't just implant it right away, as he still needs the sharingan in his right eye. Why? Izanagi.
The Sage of Six Paths divided the ten-tailed beast into the nine tailed beasts with Izanagi. Therefore, in order to bring them back into one, Madara needs to use Izanagi. This also explains why he only needs small parts of the eight and nine tails. He will take the essence of their chakra and anything else that was sealed, and multiply it (with Izanagi) to create enough eight and nine tails to fully recreate the ten-tails (it's Izanagi, it can do anything!). Then he'll replace his sealed eye with the Rinnegan, restoring him to Sage powers. Almost.
Before he can do any of this, however, he needs to get the other three paths (he has Senju, Uchiha, and Uzumaki. He's missing three). When he get's these three paths, and enfuses his DNA with the missing parts, he will basically have the DNA of the Sage. This is important, as he needs to be as close to the Sage as possible in order to be the Juubi's jinchuriki.
The Juubi is much more powerful than any of the other tailed beasts (as it's a combination). This means that it takes a great power to keep control over it (ie. the Sage). Having the Sage's DNA will give him close to the Sage's power (as simply having the 1st's DNA implanted in them gives the user increased physical abilities, so on and so forth), and with the Rinnegan will allow him to be the ten-tailed host. He can't use Izanagi to recreate the ten-tailed beast until he can get the paths, as if he recreated the Juubi before he could be it's host, it would run rampat and destroy everything.
This theory also answers another common theory. "Will the statue be the new body of the Juubi?" Yes and no. Madara will use Izanagi to reshape the statue into the true form of the Juubi, with the sealed beasts within it, thus giving it the soul/chakra of the Juubi as well.
But wait, he has one sharingan left, and Izanagi lasts one minute per eye. How will he do all of this (turn the statue into the body, multiply the Hachi/Kyuubi, recreate the Juubi) in just
one minute? The answer lies in the other three paths. Izanagi is first initiated through having Senju and Uchiha DNA (two of the paths). Having more than two of the paths may increase the abilities of Izanagi, allowing Madara to use it in a larger time frame, and in a large space (not just his personal space).
Now, with "What happened to the other Rinnegan?", "Will the statue become the new Juubi's body?", "Why did Madara only need little bits of the eight and nine tails?" and "How will Madara become the Juubi's host?" answered, there are a few more questions we need to be answered. "Who was in the last coffin?", and "What are the other three paths?". I don't know the answers to these, but at least I've been able to answer a few questions (most likely).
Ultimately, I'd like to ask one thing of you. After reading all of that, what do you think? Ask questions and pick apart my theory, I'd like to see if I can defend it.