This is the theory that I've been working on for several weeks. This theory uses only information that we've seen directly in the manga so far.
Madara wants to form the juubi and become the jinchuuriki of it to gain ultimate power. But, what power exactly? Nagato seemed to have one plan, but Obito has a completely different plan. Both these plans originated with Madara, so he must have lied to at least one of them. I think he lied to both. Let's review history by picking apart some major events and changes to the "plan" that we know about.
WARNING: There is a lot of text to read here. I've tried to break it up and hide parts of it. If you want a Tl;Dr summery, I added one at the bottom.
Madara's Original Plan
In the time before Madara met Obito, Madara created Akatsuki and gave the Rinnegan to Nagato. However, we know that Madara was not fully immortal and at some point Madara needed to die and be resurrected. The original plan was to have Nagato resurrect him.
This is why Nagato got the Rinnegan, and why Madara spent so much time manipulating and guiding Nagato. Madara had to die, and he needed somebody with enough power to continue the plan and then be able to resurrect him at the correct time.
But how? How does Madara convince Nagato to resurrect him when all the bijuu have been captured? The answer is in Nagato's plan: Nagato wanted to use the Bijuu to create a weapon. Which of Nagatos's ninjutsu techniques, if they were super-bijuu-powered, would create such a weapon that would be able to destroy nations? Shinra Tensei could, or chibaku tensei could too, but Nagato's weapon doesn't seem to be super-powered versions of either of these. Plus, Madara MUST have convinced Nagato that he was the key to creating this weapon. Nagato would resurrect Madara and Madara would turn the Bijuu into a weapon. I'm going to call this plan "Ultimate Amaterasu", although that title is a metaphor. The reference is to the fact that Amaterasu is the primary offensive weapon of Madara's sharingan eyes, a power that Nagato did not have. A version of Amaterasu with EMS eyes and the full chakra of the Juubi would indeed be able to wipe entire nations off the map and burn them forever.
IMPORTANT: Nagato would not agree to resurrect Madara if they had two different plans. Madara MUST have told Nagato that he had the same plan (creating the ultimate weapon) and that Madara was the only one who could make it. There's no other way to convince Nagato to resurrect Madara. If it was something Nagato could do himself, Madara would never be resurrected.
Nagato wanted to use the Bijuu to create a weapon. However, Nagato NEVER mentions the Juubi. Nagato only mentions the Bijuu. Nagato doesn't seem to know that they can be combined into the Juubi, and never describes exactly what this weapon would be. This is because Nagato does not know. This is where Madara comes in. When the Fox is captured, Nagato is supposed to revive Madara to create the weapon. This is a LIE. Madara is going to combine the Bijuu into the Juubi, become the Jinchuuriki of it. At this point, he probably kills Nagato, gets his original rinnegan eyes back, and does something crazy.
Nagato learned about this plan because Madara told him, and Nagato agreed to help with it because Madara was the only person who could do it.
If this doesn't make sense, remember this: Madara didn't plan to meet Obito. That was an accident. Madara told Obito to resurrect him. If Madara didn't want to be resurrected before, why would he only want to be resurrected after meeting Obito? The answer is that HE ALWAYS PLANNED TO BE RESURRECTED, and his plan was basically the same whether he met Obito or not. Meeting Obito was an ACCIDENT, so it must not have changed his goals, only the the way those goals were achieved. Obito gave Madara a Plan B, but didn't change the goals at all. If Madara wanted to be resurrected after meeting Obito, he also wanted to be resurrected BEFORE meeting Obito, and the only person available to do it was Nagato.
Also remember: Nobody ever mentioned Infinite Tsukuyomi before Obito said it, and Obito never talked about it with Nagato. Obito had a different plan from Nagato, and both of them thought Madara agreed with them.
Madara Meets Obito
Madara's original plan has a single point of failure: Nagato. If Nagato dies, or if he changes his mind, Madara never gets resurrected. When Madara met and manipulated Obito he got his Plan B. If Nagato died, Obito would steal the eyes to resurrect Madara himself. If Nagato changed his mind, Obito could control him with genjutsu. In either case, now Madara has two points of failure, which is good.
Obito doesn't want to create a weapon, he has his own plan: Infinite Tsukuyomi. Obito is inspired by the death of Rin to end the world and turn it into a permanent dream where dead people are alive and everybody is happy. This is the plan that Madara told to Obito, to manipulate Obito into following Madara.
Obito is a nihilist and wants to turn the world into a dream where everybody (Rin) is alive and everybody is happy. Obito wasn't scarred by violence in war like Nagato was. Instead, Obito was traumatized by the death of Rin. Obito has different motivations from Nagato, and Madara manipulated him differently by telling him about Infinite Tsukuyomi instead of the chakra weapon that he told Nagato. However, Obito's eyes can use Kamui, not Tsukuyomi. This means Obito MUST either resurrect Madara or, as plan B, force Sasuke to do it instead. Madara doesn't know about Sasuke because Madara was dead before Sasuke was born. This part of the plan was Obito's only.
Emergency Change of Plan: Kabuto
Obviously Kabuto wasn't part of the plan, and Kabuto resurrected Madara with Edo Tensei before Obito could resurrect him with Rinne Tensei. This is why Obito didn't create the Juubi immediately after collecting the two treasures from the battlefield: He needed to wait for Kabuto to either be defeated, or for Sasuke to recover from his surgery. After Kabuto was defeated and Edo Tensei appeared to have been ended, Obito tried to create the Juubi so he would have enough chakra to revive Madara (again). Luckily for him, Madara wasn't released and does not need to be resurrected again.
Madara's "Real" Plan
Nagato thinks Madara wants one thing (Ultimate Amaterasu). Obito thinks Madara wants another thing (Infinite Tsukuyomi). Both of these people were trusted by Madara to make sure he was resurrected. Nagato had the Rinnegan and Obito had powerful genjutsu capable of controlling Nagato if necessary. Both these two were manipulated with different goals in order to make sure they resurrected Madara.
Notice that Tsukuyomi comes from Madara's LEFT eye only. Amaterasu comes from the RIGHT eye only. Each of these represents half of Madara's true power. This is why I called Nagato's plan "Ultimate Amaterasu", because of the symmetry. Nagato wants to create an offensive weapon from one of Madara's eyes (Amaterasu). Obito wants to create an illusion from Madara's other eye (Tsukuyomi). Each of these only requires one eye from Madara. His true power (Susanoo) comes from both eyes at the same time. Actually, because of Hashirama, his "true power" is much more than that.
We've seen Madara's stabilized Susanoo: It's a mountain-sized warrior made of stabilized chakra, but that warrior is form only (Spiritual Energy). Madara has inherited the spiritual energy necessary to create Susanoo through his ancestors from the Sage of 6 Paths. We also know what happened to the physical energy of the sage: It was inherited by Hashirama. Madara now has Hashirama's power.
Remember the old story about how the Sage created the Bijuu using Izanagi: Spiritual Energy created form, Physical Energy gave that form life. One creature (the Juubi) was turned into 9 separate creatures (the Bijuu). What parts of this equation does Madara have now? He obviously has the spiritual energy ability to create a form (Stabilized Susanoo). He has Hashirama's power to turn chakra into life (wood release). He has the Sage's Rinnegan. All he needs now is the chakra of the Juubi and he can use Izanagi to create something new.
Keep in mind that on several occasions we've heard this: Konan said that the combination of Uchiha and Senju power was "the power of the sage". Both Madara and Obito have suggested that the combination of these two powers would lead to the power of the sage. In the most recent chapter Zetsu says that combining these two powers would lead to a new power that had never been seen. Even Kabuto seemed to think that the real power of the sage was greater than Obito's plan. But, if you look at the infinite tsukuyomi plan, it doesn't require Senju power at all: Madara, a pure-blood Uchiha, uses an Uchiha technique (Tsukuyomi) to capture the world in a genjutsu. Where's the Senju power come in?
It stands to reason that Madara's real plan is going to utilize these powers that he has collected. I'm calling this plan, again to keep up the metaphore, "Living Susanoo". It's the plan that requires both Madara's eyes (Susanoo) and Hashirama's physical energy/wood release powers to create a new living entity from the power of the Juubi. Madara, in essence, becomes a gigantic monster like the Juubi and is able to conquer and even destroy the world.
Tl;Dr - Summary
1) Tsukuyomi requires only one MS eye. Madara has two MS eyes, and has actually leveled them up to EMS and then Rinnegan. He has more eye power than the infinite tsukuyomi plan requires.
2) Infinite Tsukuyomi only requires Uchiha power. Madara has Uchiha AND Senju (Hashirama's wood release) power. Madara has more Senju power than the infinite tsukuyomi plan requires.
3) Madara clearly told two different plans to two different people (Nagato and Obito). Both of these only require half of his power.
4) Madara's "real" plan should use both his eyes (Susanoo), and should combine the spiritual/Uchiha energy with the physical/Senju energy, since he spent a lot of time and effort to gather these things.
5) Izanagi, the power of the sage to combine spiritual and physical energies, could be used to turn Madara's stabilized Susanoo into a gigantic monster with Juubi power. We know the Sage used this same creation power to create the Bijuu. Madara can use this power to create something new.
6) This theory uses only information we've learned in the manga and requires no new powers or other unexplained things. The only requirement is that you have to believe that Madara lied to Obito the same way Madara lied to Nagato. Since these two men were told different things by Madara, he already must have lied to at least one of them. I think he lied to both.