I just listened to part of the podcast, enough to hear Greg's theory and a bit beyond that. I stopped it an hour in so if anyone would like to add something, please go ahead. Anyways, I thought it would be nice to post Greg's Momo theory on here so that people can easily read it and discuss it here, without having to go through the podcast.
First off a very interesting theory that came up before Momo was discussed was in relation to what Kin'emon said about having met both Whitebeard and Roger. Greg thought that this implies that he met them at the same time, which could shed light on this scene where Roger and Whitebeard are talking: http://manga.life/read-online/OnePiece/chapter-576/index-1/page-10 What if the island they are sitting here turns out to be Wano?
Anyways, on to the good stuff about Momo. I hope I'm phrasing everything right, and if not Greg can slap me. People who thought that Momo was just being a stupid child, or lying to seem cool, were deceived by Oda. Kids normally are not trustworthy, but Momo has been used to drop hints here and there (he was humourously called "Spoilerosuke", lol). Examples include when he said that all Pirates were big, tall and scary, which turned out to be a fitting description of Kaidou. Or his fear of dragons, which is likely a hint to Kaidou's power. Or how he will be Shogun some day, which makes sense because he is royalty and we know the current Shogun will be overthrown.
So what does Momo mean by saying that he knows Roger? Greg thinks it's not that time runs differently on Wano or anything like that, but that Momo and the Kouzuki family in general have either a very special type of Color of Observation or the Voice of All Things (they were kind of mentioning both so I did not quite catch which one Momo and the Kouzukis probably have). The question is, how did the Kouzuki clan create the Poneglyphs, specifically with the ancient language on it? Perhaps they were shown the language by people from the past. We know that Momo can hear voices, as he stated when they were on the way to the whale. It ties in with an old theory about what the Voice could potentially evolve into: maybe it will not just be a voice in the end, but also a way of seeing visuals. When Rayleigh said that everyone inherently has the capacity to use Haki, even if few actually awaken this ability, Oda may have been doing more than just setting his story up conveniently so that he can give anyone Haki at any time. We know how there will be a world war at the end of the Manga. But how do you get the crucial info that Luffy & co will find at Raftel and along the way to everyone in the world to mobilise them? Newspapers are controlled by the Government and the CDs, so no way that's going to happen. So the idea is, what if there is a Goku-like situation (I did not read Dragon Ball so unfortunately I cannot elaborate on this analogy :D) and Luffy is able to convey the secret of the world to everyone through a world-wide awakening of haki and the ability of everyone to hear the transmitted voices. Maybe not everyone, as such, but at least a large portion of people.
Anyways, they also discussed potential of the Straw Hats to re-visit people from the past. Though it sounds cheesy, maybe Luffy will see Ace again for a moment through the Voice (which could later evolve to include visuals), or Bellemere can see Nami again, etc. Momo's remarks breathe new life into these theories. If he really did see Roger through the Voice, it opens up the possibility of these things happening.
Again, sorry if I did not phrase something right, I just wanted to make sure someone put it here so it can be discussed. I certainly think it's intriguing. Especially the world-wide awakening thing sounds a little stretchy to me right now, but I wouldn't exclude the possibility per se.
EDIT: might be nice to have a link to the podcast: https://soundcloud.com/onepiecepodcast/episode-411-the-whappening
The Momo stuff starts when they're almost 44 minutes in, in case someone wants to hear Greg's exact words. Although I feel like I copied quite a lot of it almost verbatim from the podcast.