Dread Pirate Idea

So, I was thinking of asking for this (for the sake of a plot, and for the sake of adding a cool character idea). The story of the Dread Pirate Roberts is that there is no real Dread Pirate, but only those that hold the title (and the marines) would know this in order to keep the legend of a 500+ year old pirate alive. Whenever they are looking to retire from the shichibukai and piracy, they simply seek out a protoge to be the "Cabin Boy" and start training them and teaching them, eventually, they sit back as the first mate, starting to call the new one "Roberts" and the rest of the crew all doing the same when they dock from their final voyage, to hire a new crew, and start again from the top.

Could we make a character like this in to the shichibukai? I think it would add for an interesting dynamic to have a nearly half-a-millennium old character (that isn't really)

I quite like this idea and would very much like to embrace it, though we may have to add a darker twist to it to make everyone's belief in this entity more convincing. Instead of having old crews retire amicably, the World Government has them all executed and bring on another crew who is completely clueless of the fact that "Dread Pirate Roberts" is only a mirage. I feel as though not doing this would leave too much room for mouths to blab, y'know?

There have been some pretty weird Devil Fruits, like ones that change people into toys or steal their shadows to put in dummy bodies.  I'd say an NPC with an ability that enforces silence by contract or a  an imprinted sigil that punishes someone if they try to divulge secrets wouldn't be too silly, although definitely out there (I think something simpler like hypnotism would be better there, memory stealing and such.)

This definitely sounds like something down the World Government's alley though.  Most people are just concerned about their own island. While we readers get to see all the horrible shit the World Government has sanctioned thanks to both omniscient and pirate PoVs, terrible tales about Dread Roberts wouldn't be hard to maintain since Lucci already exterminated towns. Could easily blame that on pirates to instill fear where applicable.

Agreed completely with your sentiment towards the World Government's capabilities. As for the devil fruit suggestion, it would have to be justified somehow that this devil fruit (whatever it may end up being) has somehow been in the possession of the World Government. Could be interesting in its own regard, but I'm not sure we wanna stretch this into that big of a plot point.

My concern would be, would every current "Roberts" know that the people they have been sailing with for the past thirty to fifty years is about to be killed except their favorite? Unless they are picked off slowly by having CP agents or something similar tail them to execute them if they are about to blab, or thought to blab?

It would make sense for the chosen one to be aware of this bloody history. Kinda reminiscent of how Fuhrer King Bradley is raised in FMAB.

That--is a very good idea. Maybe the rest of the crew is casually reminded of it, so that they are aware of what awaits them should they even think of blabbing? However, you are the boss, it is up to you sir! I'd just like to have one of my characters be the "next selection" for the title, only to make their escape to live their own life.