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Before I proceed with the actual speculation, I would like to clear the meaning of “World Government” for upcoming thoughts. As we know, the political organization is lead by the Gorosei, the “Five Elder Stars”:

They do not only hold the greatest power in the One Piece world commanding all the Admirals and Vice-Admirals, Kong, Magellan, the latest Pacifistas, groups like CP0 and – probably the guy with the most potential at all – Dr. Vegapunk. On top of that, since they are every non-pirate’s masterminds and even look like they would have serious fighting skills, they might be on a level with the very strongest – possibly on par with a Yonko’s strength each (- just imagine the upcoming war with four Yonkos and the Pirate King versus the five Elder Stars (alongside with all the other guys)).
According to this (and ignoring those marionettes called “Tenryuubito”) we might reduce the meaning of “World Government” to that geezer team without eyebrows. So to say, five guys with one aim.
Therefor, I want to discuss the question:

The great pirate era started with Roger’s last words and was refreshed by Whitebeard as he confirmed the One Piece to be real:

So we could assume that along with the pirates the WG wants to find the One Piece. But I suspect it to be not the whole truth. The One Piece treasure will only be a means to an end. Actually, it could not be simpler:

As obvious as it seems, since every living being (without Kaidou) wants to survive, it gets quite a different meaning if we consider how hard the WG already tried to do so. It is not only about defeating and imprisoning some pirates so concrete danger dissolves. The WG is in charge of possibilites “to become as immortal as possible”.
It probably started long time before Gol D. Roger and the following pirate era.
It is all about living forever – meaning they will not die by age, nor by any disease (or wound).

We do not know how Jewelry Bonney is related to the World Government. But we do know that they are interested in her:

We also know that she is able to manipulate people’s age:

Thus, she is pretty useful to reach immortality at least in one aspect – those who benefit from her power continually would not have to die of aging. But even if the fruit’s effects worked permanently, it would not be a durable solution. The effect could be revoked as the devil fruit user becomes unconscious or dead. Or just resists the WG’s will to help.

Law’s fruit was considered to be the ultimate fruit in the world:

Eventhough, we do not know how it works exactly. But we know that the user could give his life in order to make somebody “ageless”. Still, many discussions have been arisen what it could mean. Some speculate the benefitted individual does not age anymore, some even say he/ she/ it could only die by age, but not by wounds or diseases. (In the last case, Trafalgar Law’s and Bonney’s abilities combined could make the benefitted one truly immortal as long as “Bonney’s” effect does not get revoked (and after years renewed) – which made it imbalanced.)

Princess Mansherry’s Devil Fruit makes her able to heal wounds:

Its true potential is still unknown, e. g. if it was useful for (deadly) diseases, too, or if it could even revive (!) at its potentials pinnecale. Nevertheless, it also only prevents from dying indirectly. With Law’s Fruit being considered as the most valuable DF, I am not so sure about if the WG knew about Mansherry or the Chiyu Chiyu No Mi. Sengoku seemed somewhat amazed – as if he did not know about the fruit before:

Who knows – maybe the fruit’s ability will be reported to the WG by a “bad Marine” (like Bastille or so) or the CP0 meaning that Mansherry could get captured during the upcoming Reverie.

Even lesser is known about Kaidou. It was stated that he could not die, yet:

The WG’s dream seems to be his curse. Until now, it is not known, why and how he survived so far. It might be related to his possible Devil fruit or – and that would be inaccessible by the WG – just fate and/ or luck. (In case preordained “fate” existed in the One Piece world, the WG’s efforts would be useless either way.)

Ceasar Clown revealed that Dr. Vegapunk experimented to turn humans into giants:

We know from SBS Vol. 19 Ch. 168, p. 46 that giants are able to become way older than humans – up to 300 years:

Of course, obtaining a giants age would not lead to immortality, but it would buy some time if they could copy that effect of slow aging. Also, if the WG got to know why giants become that old, they might find a way to become even older.

As recently shown Dr. Kureha is quite vital at 141 years.

As we know, she always spoke of herself as a young woman. Now imagine if she would have found the secret to age like a giant. She would have been aged like a ~65 years old “normal OP-human”. She also will attend the Reverie. Maybe Wapol told information about Dr. Kureha to the WG in exchange which could lead to a capturing, too. Or maybe Dr. Kureha does already know Dr. Vegapunk. We will see – but it is for sure that her biological clock would be interesting to the WG.

Beyond the pirate theme of the 16th century from our real life, One Piece is influenced by futuristic science fiction aspects, too. Besides Dr. Vegapunk (and Franky), also Dr. Tsukimi seemed to work on “robotic” individuals.

In parallel to other experiments, Dr. Vegapunk created cyborgs. Cyborgs consist of biological individuals with mechanical parts:

At first glance, they just seem to be powerful military forces. But there could be a deeper meaning to their nature. If it was possible to replace every single (human) body part step by step, biologic individuals could be prevented from dying by age, diseases or wounds – as long as they are able to conservate the (human’s) conscious mind.
Until now, they seem to have failured in “recreating” a human being, since “Kuma” lost his personality:

But it could only be a matter of time, until Dr. Vegapunks finds out how “to replace a brain with its personality”. An important clue is that Dr. Vegapunk managed to transmit a Devil Fruit onto a lifeless object plus that Devil Fruits might transfer characteristics of the previous owner(s).

Not being a great military force, the automatas that Dr. Tsukimi build seem to have emotions:

Therefor Dr. Tsukimi seems to be the first one shown who created entirely artificial life:

Also, Enel just had to “breathe life into automatas” by electric energy “to (re)vive” them:

Using this very technology could mean that Dr. Tsukimi found a way to play God. Still we do not know where Dr. Tsukimi lived or to whom he could belong to. But maybe his residence was close to the Marines/ WG:

The question would be, why the WG did not use Dr. Tsukimi’s knowledge, yet. Probably, the Marines/ the WG did not know about him/ his knowledge/ what Automatas are. (But I guess Dr. Vegapunk most likely will or even does already know.)

Last but not least, I will link the WG’s goal to the big treasure One Piece which might be the only durable solution to survive.
Referring to my main theory

(- that OP’s loose template is based on “The Mysterious Cities of Gold” from 1982), the One Piece will be a device which bears the energy of the inner Sun (- besides countless (loose) parallels, Sun energy was once used peacefully by two kingdoms, but then a war broke out, Sun weapons were used and the kingdoms vanished). In TMCoG it was stated that it provides those who use it as a medicine that could cure any disease. Everybody could live eternally.
If that was the case, the WG would be highly interested in that device as well. But that is only speculation..~
(Also, the Gorosei could be aliens.)

At this rate, it seems that the Gorosei (= masterminds of world politics) are looking for different ways to evade death due to age, diseases and/ or wounds (at least temporarily) – …

• … either by Devil Fruits (of Law, Bonney, Mansherry(, Kaidou), …), …
• … or by biology knowledge about giants and/ or Dr. Kureha, …
• … or by advanced technology regarding cyborgs and automatas and the “One Piece”.

Nevertheless, nothing and no one seems to be invulnverable so nothing or nobody could become immortal, technically (- Kaidou was defeated before, still seems to have “luck”; Poneglyphs can be formed). But still, they try to survive eternally, thus becoming “as immortal as possible“.

*Theory by EddWarD.NewGate

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