ONE PIECE GOLD
I believe that by the end of the series, there will still be a Monster Trio, only it’s members are not those whom we usually assign the title to. With the drastic increase of force the Straw Hats are likely going to gain in the future, whether it be Sub-Captains or simply more core Straw Hat Crewmember, Luffy’s forces are certainly upping their game. While Zoro and Sanji will undoubtedly remain his right and left-hand men, the Straw Hats will gain such notoriety “monstrous” strength, that designating the term will undoubtedly become rather nullified. However, the legend of the “Monster Trio” WILL carry on. It will carry on via new flag holders, the trio that coined the term in the first place. Nami, Ussop and Chopper. Why? Allow me to explain.
Once upon a time, the six members of the Straw Hat Crew were divided in half between two trios:
The “Monster Trio” and the “Weakling Trio“.
This was something the weakling trio themselves took on board, accepting the fact that their fellow Crewmembers possessed outrageous and abnormal strength. The gap at that time was clear, yet we still respected the “Weakling Trio” for they at least always tried their best. One of the major ways they did this was through trickery and deceit, as well as entertaining dumb luck (a theme recurrent in the Straw Hats), in other words; showmanship. Even Chopper would get the edge on opponents by the unpredictability of his abilities.
Now, move on two years, and they have more Crew mates, the power levels have shifted somewhat and due to varying battle styles, it has become harder to determine who is superior to who within the crew. There are obvious things that some succeed in compared to others, but I like to think that sometimes the Straw Hat Crew is one big rock-paper-scissors collection. They all have their strengths and weaknesses. However, it is only now that the Luffy-Zoro-Sanji Trio are beginning to finally have allies of a comparable power level. This broadens the terminology even further. The “Monster Trio” title is fading, but Luffy-Zoro-Sanji shall always remain a unstoppable combo.
But if the “Monster Trio” title is fading, then to me the “Weakling Trio” title is dead and buried. Or at least will be very very soon. These characters have grown up a lot since they were introduced. Yes they are the same characters with many of the same faults, but they demonstrate those faults less and less and they have developed their skills enormously, with plenty more room for improvement. Even considering that Chopper probably eclipses Brook regarding raw strength these days only makes my rock-paper-scissors concept all the more relevant.
Section Conclusion: Calling Nami, Ussop and Chopper the “Weakling Trio” after the progress they’ve made, and comparing them to Franky, Robin and Brook is unfair. On the other hand, they will always probably hold that title in our head due to the many years of that being the case. That is, unless Oda does something about it.
First let me predict several developments that Nami, Ussop and Chopper will receive at sometime in the future.
Nami: So far in the series we have witnessed two characters who showcase obvious signs of witch lore.
These are Big Mum and Katarina Devon. I shant go into too much detail regarding these two, considering there are several theories concerning these facts and i’m sure many have made such connections already.
To keep it simple, Big mum has been related to the witch of Hansel & Gretel concerning her candy obsession. Her flag has a branch on it, linking her to paganism. Her facial features match the grizzly classic representation of witches and given her size, flight seems her most likely form of transport. Her ship sings and her subordinates wear magician outfits, enough said.
As for Katarina, Devon relates to the Devon Witches, a famous historical witching incident. Her weapon links her to the moon, something corely associated to paganism and witchcraft and just like Big Mum, her features are damn witchy. More significantly, Katarina is on Blackbeard’s crew, and whether or not they end up fighting, it is true to say that Luffy and Blackbeard’s crew seem to match up in more ways than one.
Now, it is crucial to understand that witches often appear in covens, typically of three. So far in One Piece we have encountered other females that have been compared to hags and witches, but none that fully satisfy the connection of a Pirate Coven Trio. Dr. Kureha is not a Pirate and Bonney isn’t witchy enough for me. Nami on the otherhand…
Putting past the simple fact that she is a clever and resourceful young girl, a situation that would be enough to condemn you for witchcraft in some periods of time, Zoro has even called her a witch in the past. Getting juicier. Nami studied on Weatheria, and island containing scientists dressed primarily as WIZARDS. Her weapon itself is comparable to a staff, or when disconnected into three, even a wand.
The biggest clue however is when on Fishman Island, an enemy cried out her powers were magic, yet she immediately declared them to be scientific. It would be of typical One Piece fashion to change Nami’s bounty name to be both something funny yet relevant to the series. As such, I believe that Nami shall no longer be called “Burglar Cat” Nami by the Marines, and instead be called “Sorceress” Nami. Thus making her the final witch of the coven.
Ussop: I n fact have next nothing to be said about Ussop that hasn’t been said before. He will likely grow in strength, courage and skill, as well as further develop his Haki. However, let us not forget where Ussop’s origins lie. He shall forever be the master of trickery in the Straw Hat crew, and I believe his bounty and and bounty name shall reflect this, especially after the whole “God Ussop” situation. 
I can’t predict what these will be, but I just want remind everyone that Ussop is indeed the Master of Showmanship! (I’ll explain in Section III)
Chopper: Chopper Chopper Chopper… Easily the most physically strong of the Weakling Trio, he is currently in my eyes the closest to pulling away from the title with Ussop in hot second. Just like the other two, he is often underestimated due to appearance, perhaps the most so, and his opponents pay the price.
I believe that Chopper may well be the first Straw Hat to unlock his Awakening, as I believe he is the one who has experimented with his Devil Fruit the most, even more than Luffy. Once he does, his upgrade will be phenomenal, and his Awakened form potentially more terrifying even than “Monster Point”.
Speaking of Monster, isn’t that the name that Chopper has been called ever since he’s been introduced. Isn’t “Monster” the word that has always clung to Chopper, only for him to embrace being one for the sake of Luffy and the Crew?
I believe that once the Marines know Chopper isn’t the “Cotton-Candy Eater” they know him to be, his new and rightfully deserved Bounty poster will read “Monster” Chopper.
And what better place for a Monster…. than in the “Monster Trio”?
Here my theory comes to its climactic cultivation. The point that I predict all these pieces of information will collide.
At some point in the One Piece future, I, and many other fans predict there will be an arc featuring the Emerald City. Who resides within the Emerald City? Why, the Wizard of Oz, an all powerful, all seeing, all phony wizard who is in fact.. yes, a phony. But what a grand impression he made! Using stage effects to create a colossal version of himself capable of amazing spells and feats!
Kinda sounds like something Nami, Chopper and Ussop would cook up to get out of a tight spot right? Right.
Nami represents the Witch, but lets not forget her “magic” is science (and thus metaphorically speaking stage effects), and she was trained by wizards! Even her staff hints to the fact that she is witch with a wizard’s magic. This will also be Ussop’s master piece, the ultimate piece of trickery prior him becoming a truly strong warrior of the sea. And of course, it’s Chopper who will be the all powerful being. The Monster. My prediction is this:
There will come a moment within the Emerald City where the Straw Hats require assistance from the “Weakling Trio”. Nami shall use her illusionary mirage techniques to create a gigantic version of an Awakened Chopper. Chopper shall pretend to be an all powerful godly being, pretending to create spells which Ussop shall make occur using his sniping skill and inventiveness. Ussop shall also be the announcer, distorting his voice via Dials to claim to be some sort of God.
This monstrous apparition, upon discovery, shall cause the World Government to label these three Straw Hats as the “Monster Trio”.
I believe this is fitting for the reasons that, as stated earlier, these three rely on stage they are presented with to win their fights more than others. They use showmanship and intimidation to win battles more than others. To gain infamy in such a way as this fits perfectly with their characters, even the fact that Chopper is perceived as some sort of divine monster is perfect.
While all other Straw Hat members will gain further infamy in unrelated ways, this seems a perfect way for the new Monster Trio to earn awe from the world. It will consist of a Witch, a God, and of course a Monster.
*Theory by The クンフー ジュゴン