Before anybody says it, I know full well that Limbo is a Rinnegan ability. I just think it's stupid that it is.

Let's be honest, Limbo as it is currently constructed is a bit of a plot hole. Why can only Madara use it if it is a Rinnegan ability? Some people may argue that it is his unique ability, just like Sasuke has a unique Rinnegan ability, but first of all, Sasuke's Rinnegan is clearly not a normal Rinnegan, it's a Rinne-Sharingan, so to use that as a basis for what to expect from normal Rinnegan's isn't wise. Not even the Sage of Six Paths has a unique Rinnegan ability, why would Madara? Second of all, it being Madara's unique ability doesn't answer the question, because Nagato should be able to use it since he has Madara's eyes. He can use every single other one of Madara's Rinnegan abilities, except for this one. If Kishimoto had instead made it Madara's Mangekyo ability. While it still doesn't make massive sense, it is at least consistent with the fact that Nagato can't use Madara's Mangekyo Sharingan or revert his Rinnegan back down to it. Also, it would give a very simple explanation as to what Madara's Mangekyo ability is. Limbo is a pretty swole ability, befitting of the man whose name is synonymous with power. It would also explain why he never used it against the 5 Kage. Madara was trying to have fun, what fun would it be fighting people with an ability they have absolutely no chance of overcoming. It would also explain why Madara uses clones so much in his fighting style, because it is quite bizzare that a Senju technique was so integral to his arsenal. Maybe, Kishimoto could have said that, just like Hashirama's wood clones inspired Tobirama's shadow clones, Madara's limbo clones inspired Hashirama's wood clones.

However, I do see the main reason why this would be a bad idea. If Limbo was Madara's Mangekyo ability, there would be no way to explain logically how Hashirama beat him. Hashirama is completely ill-equipped to deal with Limbo. He wouldn't be able to see or sense the Limbo clones because he has neither the Rinnegan nor Six Paths Sage Mode. Explaining how Hashirama won a fight against Madara, Kurama and up to 5 completely undetectable clones of Madara that are essentially as strong as him would be essentially impossible. Although, a pretty easy explanation for Kishimoto would be that Hashirama's cells and the consequent Rinnegan buffed Limbo to only be beatable by other six paths characters. So maybe, originally, you only needed to have a Sharingan or Sage Mode to see or sense Limbo, but once Madara reunited the chakra of the Sage of Six Paths within himself, it got knocked up a level to the Rinnegan and Six Paths Sage Mode. That way, Madara could be beatable for Hashirama and it would give a very clear and solid reason as to why Hashirama learned Sage Mode (aside from the obvious reason that Sage Mode makes you more powerful).

Rant over.