We've seen vending machines. We've seen slimes. We've seen bugs. We've seen prostitutes.

These aren't weird enough. How about:

Reborn Into Another World As A RTX 3080 Graphics Card And I Compete In A Spelling Bee Competition With AMD Processors.

Reborn Into Another World As An Empty Can Of Pringles And I Enter A Competition To Become The Champion To Defeat Doritos.

Reborn Into Another World As A Breast Implant And I Compete With Dildos To Become The Best Rated Adult Toy In The Universe.

Reborn as a post on Royalroad.com in the thread, "What is the weirdest thing as which you could be reborn into another world and then enter a tournament arc?" trying to become the weirdest post in the thread.

Go planaria or go home.

Reborn as a severed toe, forced to compete in a beauty competition

Reborn as a meson and enter a competition to defeat the God Particle. 

  Yay science!

David Wrote: Reborn as a post on Royalroad.com in the thread, "What is the weirdest thing as which you could be reborn into another world and then enter a tournament arc?" trying to become the weirdest post in the thread.

No one can be this meta. It is not possible. It is not done.

Reborn as a quantum scientist, entering a tournament against the observer effect and the double-slit experiment. 

Really, I don't think it gets weirder than quantum physics. More random, quite possibly, but not weirder. 

Oh, wait. Reborn as a sex therapist in a fighting tournament with an 80's BDSM/Frazetta aesthetic. That could get up there pretty quickly. 

Reborn as a particle in a tournament on who can stay unobserved the longest.

Reborn as an alternate version of yourself, doomed to fight all other versions of you to the death. Title: My Multiverse Mutilation Match

One where reincarnation fails and you are reincarnated dead. then the story just ends. The tournament gets played normally by everyone else with no upsets, other than the world being destroyed because the main character is dead. the main character then becomes death and kills everyone in every story and real life. For no particular reason.
Edit: I am become death, destroyer of worlds, literally. Just to add a little of the science everyone loves in, too. If you don't get it, you don't get it. if you do, congratz to you! 



Clue: it has to do with a quote from hindu scripture  used in a very famous test .
Edit 2: Reworded this

Okay, so I was gonna make a joke about a gamer being reincarnated as an NPC or as someone else's player character, but a thought crossed my mind.

Is the film Gamer (2008) an Isekai?

I had a dream one time where I was a dolphin with a human rider competing in something like the equestrian Olympics events.  Only, y'know, mostly underwater.

Reborn as an umpire, destined to travel across dimensions, competing with other umpires to survive arranging more and more challenging tournaments.    See the MC adjudicate the off-side rule during an centaur football match, then level up until he can disqualify a cheater in an a game of intergalactic bar billiards, and face down an over powered Chūnibyō-god whose scream as his power hits 9000 is disturbing the caterers and raising complains among the groundsmen maintaining the amphitheatre.

I was reborn into a world as me but 3 inches shorter and I have to do pretend like I am holk hogan to stop the demon king

I have a draft where a WW2 tank gets magically transported to an ice cave and a little girl brings it to life by dropping a magic crystal inside it when she hides inside it escaping a group of bandits she stole from.

Technically its canon to the same universe as my stories. I just never got the chance to work on it further.

Reincarnated as a slice of pie. Everyone wants to eat you, your only power is communicating telepathically, and you must rely on your wits to survive by turning your enemies against each other. 

When you finally reach the finale where it's only you and your final opponent left, the only way is to somehow convince them to forfeit. Then you win the tournament. You only ever sat there as you have no limbs, and the audience is very confused, you are probably also very confused, the gods themselves may be confused indeed, but through guile and trickery, somehow you won. 

Reborn as a magic strand of hair , everyday you teleport onto a new host in a world : where they are told about the miracle hair that grants a superpower if you can manage to pluck / kill it.
Its proven true by super powered people also looking for "that hair"..come the next day who knows where the hair will be ?

Could get somewhat dystopian with daily body checks and all sorts of nonsense.

Once I saw the "Reborn as Autonomous Machine Arsenal" (or something kinda that) and it was lit.