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So, this has been on my mind for awhile. I think that especially after the timeskip, 

Robin is one of the biggest examples of a previously strong character falling to the wayside in One PIece. When she's first introduced, we find out that she has a bounty nearly as high as Crocodile's, she's the number 2 of Baroque works, and she's essentially one of the most dangerous villains we've met. She's extremely calculating, observant, and has a devil's fruit that works handily against any type but a logia. One could argue that Robin's bounty reflects her knowledge as a survivor of Ohara rather than combat threat. To that, I'd like to remind everyone who easily she beat Pell, who survived a HUGE destructive bomb and was one of Alabasta's elite royal guards.

Pell was considered Alabasta's strongest warrior, and Robin defeated him in a matter of seconds. 

Strawhat Bounties at the Time of her Joining:

1. Luffy - 100 million

2. Robin - 79 million

3. Zoro - 60 million

At this point in the story, I'm not entirely convinced that Zoro could have beat Robin, considering how swiftly she was able to take down a very strong warrior. 

After this, she was allowed to shine in Skypeia, and had a few moments, but she was largely not instrumental in taking down any bosses or top-level officers the way that Luffy, Zoro,a nd Sanji were. She was still depicted as kind of a part of the monster gang in the Thriller Bark Arc with how unafraid she was, but  Robin's fighting capabilities (the woman specialized in ASSASSINATION) have largely been de-emphasized since at least Skypeia. Moreover, I don't think anyone would argue that she kept pace with Zoro, someone she arguably may have been able to beat when first introduced.

Post-time skip, Robin has learned some cool new abilites. She can now make gigantic limbs and even a clone of herself. Except... the strength of her ability has most often been the element of surprise and the ability to infiltrate the enemy on their own body, stripping them of weapons, putting them into a submission hold, etc. I don't think these abilities are completely useless, and Robin can obviously stand up to grunt soldiers in Fishman Island, BUT she is nowhere near the frontline fighter that Zoro or even Sanji are. The most she has done with her powers post time skip is keep the Sunny from crashing and protect Rebecca from Diamante while Kyros fought him.

What I would have liked to see after Robin spent TWO YEARS with the revolutionary army, the most wanted criminal in the world who likely has some of the best haki in the world (Monkey D. Dragon) and an obviously superior haki user (Sabo) is for Robin to have learned haki. Imagine if she could grab logia users into a submission hold or if she could locate stealthy targets with observation haki and no longer need to see them to sprout her limbs on/around them. Robin's fighting ability would be so much better, without endangering the balance of the crew or making her stronger than Luffy or anything. She would simply be able to hold her own better in the new world and truly have the fighting ability that the archaelogist of the future pirate king's crew should have. 

Some may argue that Robin doesn't need to be a fighter. She now has a strong crew to take care of her, and her main role is an archaeologist. Robin will always be instrumental to the crew for being the only one able to read the language on the poneglyphs left alive in this world. But she is facing big threats now.

Who is to say that one of the Yonko is not going to attempt to kidnap her and force her into their service soon? With the crew being split up more and more in recent arcs and the Strawhats preparing to make a move on Kaido (and possibly not being able to get out of a fight with Big Mom), previously strong members like Robin, need to have a boost to their fighting abilities. I know that Oda often has an issue with having women fight unless they're forced to for whatever reason, but being a major fighter is not contrary to Robin's personality. The crew could also benefit more from the tactical abilities she displayed when she was an antagonist, and Oda would do well to remember that the entire crew needs to become strong enough to eventually fight the Blackbeard Pirates and so forth- it would be boring if the monstro trio just defeated everyone.

I truly believe that Robin must get stronger for the plot to make sense. If we've seen an entire island of Haki users who aren't super strong warriors themselves on the Isle of Women, and if we've seen Tashigi, an opponent Robin defeated with absolute EASE learn how to use Haki, it's time for Robin to learn it. I dont' know how Oda should go about it (though I think it'd be hilarious if Luffy, Zoro, and Sanji somehow realize that they need to train the rest of their crew and struggle to try and teach them), but it just doesn't make sense to me that characters who were once much weaker than Robin (like Tashigi and Coby) are learning haki, when she obviously has the discipline and grit to be an effective student. And as Rayleigh stated, anyone in the One Piece universe CAN learn haki; they just need to work on training it.