You probably shouldn't have said "all ideas". You've brought this mountain of words down upon yourself!
My personal theory is that the Ancient Weapons weren't originally designed to actually be weapons. They just got re-purposed into weapons after the fact. Because we know Joyboy had the Ancient Weapons and was planning on using them. And I think that was true. But, I think Joyboy's plan was going to be to use them for their original purpose, instead of as the weapons they became better known as.
As for what the Ancient Weapons were originally? I have a feeling they were some form of "construction equipment". Either for terraforming the planet into something more suitable for Humans, or in order to mine the Forbidden Sun at the core of the planet. Or maybe a little bit of both.
I mean, you COULD use a bulldozer, or a wrecking ball, or a drill rig as a weapon. But, that's not it's intended function. I think these things were made to BUILD something, rather than destroy it. Which feels fitting, if Luffy ends up using them.
I also feel like the names are important. They're all named after the outer planets. So, I wonder if that's hinting at a extraterrestrial origin for these?
Personally, I think it's interesting that the epithets for two of the top three of Roger's crew are also named after the outer planets. Gol D Roger himself is the Pirate King or Kaizoku-Oh. The name for Neptune in Japanese is Kaiousei, with the same "kai" for "water". Rayleigh is the Dark King or Mei-Oh. The Japanese name for Pluto is Meiousei, with the same "mei" for "night" or "dark" or "underworld". In theory someone should be named after Uranos, or Tennousei, for a "Sky King" or a "Heaven King", but that character is missing. But, whatever the case, it does seem like Oda got...two-thirds of the way of directly paralleling Roger's crew with the Ancient Weapons. Which I think could be a hint that the Ancient Weapons are actually just "misunderstood good" aligned, like the Roger Pirates themselves.
Buckle up, we're going to crazy town from here on out.
My personal theory is that Humans are not native to this World. Humans are actually the alien invaders who stole this World from it's original inhabitants. And then mated with the locals, resulting in most of the hybrid races we see today. The original inhabitants of this World were the Dragons. And the Ancient Giant's horns and eyes are the result of having Dragon blood in their ancestry. This even works for other races, like the Fishmen or the Long-Limbed Tribes. The Fishmen are amphibious like turtles, and the Long-Limbed Tribes have...long limbs, like a snake-hybrid would have. Almost every "race" in One Piece has Dragon-like reptilian characteristics. Even the Tontatta make sense, since there are some really small reptiles. (Except the Minks. I'll admit, I can't explain the Minks with this theory.)
https://www.weforum.org/stories/2021/02/new-species-madagascar-smallest-reptile-nano-chameleon/
Even the Amazons. They supposedly only birth female offspring. But, it's possible that's because they're slightly lizard-like, from having a different mix of Dragon blood in their lineage. The gender of their offspring may be determined by the temperature of the environment. The environment now is colder than it used to be, due to the Forbidden Sun being mined from the core of the planet. So, all their offspring are female, because the environment isn't hot enough to trigger the birth of male offspring. So, even the Amazons are showing signs of being hybrids with something reptile-like, like the Dragons. The Amazons are literally Dragon Ladies.
https://en.wikipedia.org/wiki/Temperature-dependent_sex_determination
https://www.ncbi.nlm.nih.gov/books/NBK9989/
https://en.wikipedia.org/wiki/Dragon_Lady
So, I think it's possible, and even pretty likely, that Humans from outer space invaded and conquered this World in the past. Then used something like the Ancient Weapons to terraform the planet to suit their own biology. And also mated with the original Dragon inhabitants of this world to help themselves adapt to the environment. While also mining the planet of it's natural resources, the Forbidden Sun. Mining the Forbidden Sun had unforeseen consequences. And the Dragon's likely fought back against the invading Humans. So, the Humans killed two birds with one stone, by trapping the Dragons in the hollowed-out core of the planet while freezing it in order to keep the Forbidden Sun in check. Thus, explaining Brook and V Nusjuro's "Frozen Underworld" powers. The Ancient Weapons, therefore, are the only way to reverse what they did. Joyboy, and therefore Luffy, need the Ancient Weapons because it's the only way to unfreeze the Dragons at the core of the planet and try to make up for how they were wronged. Luffy would set Uranos to "defrost", but Imu's got it set all the way up to "charbroil" in order to try to kill the Dragons at the center of the planet before they have a chance to wake up again.
This would also explain why we're getting repeated motifs of people like Kaido and Loki dropping from great heights trying to kill themselves, but only leaving behind a giant hole in the ground, failing in their attempt. This is Oda showing us that there's something down in the core of the planet that DID fall farther than either of those two did. And is it any coincidence that the hole that Kaido leaves behind from trying to jump from a Sky Island is pretty similar to the hole that Uranos made in the ocean after it destroyed Lulusia? "They can't even die right". I'm convinced something's still alive down there, at the center of the world. The trapped Dragons may even the source of the "desires" of the Devil Fruits. The Ancient Weapons may just be the key to the Dragon's "jailcell" as it were.
For more on this theory, look over here:
https://worstgen.alwaysdata.net/for...message-will-be-the-dragons-are-coming.55673/
Now, I'm going to talk about the Ancient Weapons, individually. And what their original function was, according to this theory.
Poseidon is basically the inherited "psychic" power to control the Sea Kings. I'll admit, it's weird that this is a mostly water-centered power. Especially if the planet wasn't as flooded as it currently was. But, my theory is that the Sea Kings were basically used as "crowd control". When you remember that the majority of the Sea Kings are in the Calm Belt, around either side of the Grand Line, it kind of makes sense. The Sea Kings are designed to help get people out of harms way, and to keep them away. They're basically living "caution tape" or a giant fence around a construction site. They just never got told to move again.
Uranos seems to be a giant sky laser. We aren't even 100% certain that what destroyed Lulusia was Uranos. But, I'm betting that it probably is, personally. We actually see what is probably the Ancient Weapons in Enel's Moon Mural (Chapter 470 and 472, specifically 472 for the Weapons). Uranos is probably the "Sun" above the winged races carrying balloons. It seems like it has a lot of points or spikes on it. My personal bet is that Uranos was probably some sort of drill rig, used to make holes to get to the core of the planet so they could mine it for the Forbidden Sun energy, as hinted by the Chapter 1138 Mural. Or it's possible it was used mostly for observation or administration. It's easier to see how a project is coming along from above.
Pluton, apparently, is some sort of a battle ship, at least according to what Franky could make of the designs (Chapter 355). In the Chapter 472 Moon Mural, it's probably the "curvy" thing with a bunch of "eyes" that's below the balloon-carrying winged races. It almost looks like it could be "glowing", to me? We know it's below Wano right now. A lot of people think that Franky used his memories of the blueprints to basically remake Pluton as the Thousand Sunny. And while I like the idea, I actually think Oda might be going someplace else with this. But, as for it's role in my "construction equipment" theory. I think this could be the heavy machinery. The thing that's supposed to do some heavy lifting. Maybe moving the "junk rock" out of the way while they mined the Forbidden Sun. (That "junk rock" they discarded eventually forming the One Piece World's many moons, possibly. Enel might just be the ruler of the ancient equivalent to a junk yard or garbage dump.) And if any of these objects has some sort of "freeze ray" that could have frozen the core of the planet, I think Pluton is the most likely Weapon to have that kind of equipment. If anything, Pluton likely has a "Freeze Ray" AND a "Heat Ray" of some sort, in order to freeze or defrost anything it needs to. Possibly as some sort of equivalent to hydraulic mining. Which fits because hydraulic mining was heavily regulated or outright banned in many areas because of the damage it did to the environment. This might ultimately be where all the water underground is coming from. The Strawhats could use Pluton to free the Dragons.
https://en.wikipedia.org/wiki/Hydraulic_mining
Here's another crazy idea. I think it's possible that there's actually a Fourth Ancient Weapon out there. If you look at the Moon Mural in Chapter 472, you actually can see FOUR objects surrounding the Sky Races. A lot of people leave out the object that the Automata themselves are left behind on. It could just be representing the Moon itself. Or it could be a "carrier" or "hangar" for the other three Ancient Weapons to fit inside. A Titanus or Tor from Power Rangers, as it were. I think this Ancient Weapon could be what the Thousand Sunny is ultimately paralleling. Because, remember, the Thousand Sunny also has a bunch of other vehicles that it carries around with it. The Seabull Waver, the Mini-Merry, the Shark Sub, the Black Rhino Motorcycle, and the Brachio Tank. Two of which can combine into the Franky Shogun. So all the vehicles that the Thousand Sunny carries would be it's "Ancient Weapons", so to speak. This Fourth Weapon would probably be named after Nibiru or Planet X. Or maybe this is literally just the Moon. Still, I think it's possible we could get this as a reveal down the road.
https://science.howstuffworks.com/nibiru.htm
And for one final thought...can they combine? Because...that would be cool. It would be cool if all the Ancient Weapons could combine together into a huge mech. I guess that would turn Shirahoshi into a mecha musume/Gundam Girl. And I'd be okay with that. No idea if Oda would actually go all the way with that. The Ancient Weapons don't really LOOK like they're designed to combine together. But, crazier things have happened. And I'm kind of rooting for it, personally.
As far the the connection with the Abyss goes, I see it more like "software" to the Ancient Weapon's "hardware". The Abyss Mark system would be the ancient technology equivalent of, say, a cell phone or Siri or Alexa. A versatile piece of software that allows you to do many things. Maybe with some AI in there. It's like someone figured out how to make "cloud storage" by manipulating reality itself. The Ancient Weapons would be the "hardware". The big equipment. The equivalent to cars, or jets, or like I've been saying construction equipment.
It kind of does make you wonder, though. Could the Ancient Weapons have some sort of intelligence themselves? I mean, if even Devil Fruits, if they are ancient technology, have some form of will of their own, could that mean the Ancient Weapons do, too? Some sort of AI Copilot to help you run itself? I know a lot of people were wondering if the "Grim Reaper" that Zoro saw in Wano was actually the "klabautermann" of Pluton underneath Wano (Chapter 1038). Maybe the "klabautermann" of different ships is our equivalent of a "AI Navigator". Maybe the Ancient Weapons each have a "spirit" watching over themselves?
I don't know. There's a lot to think about with the Ancient Weapons.
