What's a D&D game without powerful people trying to murder you anyway? Sounds boring.

Depends on the game and players. Knew this one group who became literal gods before cracking level 10, (evil party) and went about just fucking things up.

I had the idea of running a pre-written campaign completely as the book says...but with the party being overleveled to all hell and back. Like, level 20 characters, or even level 30 homebrew, in a 1-10 campaign.

Oh right, those guys. Whelp.

May you live in interesting times.

Let's do some math. 7 warlords + three admirals + I assume four emperors + I dunno, Blackbeard? Also someone else.
Candidates for the position of Someone Else: Sengoku, Dragon, Rayleigh, Gol D Roger, Joyboy

Dunno about Sengoku, he doesn't feel very PC-like.
In the source material he has not much personal agency compared to the admirals or the warlords.
And Gold Roger seems to be a little bit too big a character to be an ascended PC.
Kinda surprised nobody considered Garp though.
He seems absolutely like the kind of OP PC that retires into canon after wrecking all kinds of shit

I think Roger was already established to be a campaign NPC.
Shanks come up in Luke's bacsktory, but i don't remember if Lukee invented him or picked it from the book without realising that he was a important NPC (Like with his hat).
I'd say the 7 shishibukai, 3 admirals, Possibly BlackBeard or Ace (played by Luke's brother), possibly Garp, and maybe some of the supernovae (not all of them), or CP9/0 (we know Luke fought a guy named Lucian Robert's in Megalonia, said guy could have used his character in another tournament and got it promoted to canon)

That spot check is what you call a critical failure right?