The Changes Nobody Sees.

[NotYourTypicalMan]

Also the feudal system I came up with is based on Crusader Kings 2

I see you're a man of culture as well

 

[Chaaruzu_Corner]

I see you're a man of culture as well

When I first saw it my mind was blown xD I don't really know what nobles and stuff really meant at that point so seeing it all was great

 

[YuujiEveryleaf]

Even in my head?
Too many to ever count.

1-chapter sidesteps that turn into whole arcs.
Nameless side characters that join the main cast.
Countless characters that die, don't die, betray, no they're good.
Constant personality changes.

Where does it even stop?

 

[Moonpearl]

I feel like my question is more suitable when answering a reader who's familiar with your characters. It's kind of a way for them to see the huge difference between what is and what could of been. Since you aren't speaking to a reader you'd have to explain how things are now then talk about the differences which I guess makes it a lot less mind blowing.
That said, here's my own example.

The story I'm writing was built to fulfill a single scenario at the very beginning (that barely even ended up happening) so once I got past it I felt a little dumb because I had nowhere else to go with it. Every now and then I'd randomly tweak the storyline and characters in an attempt to create the ending, and it just startled me how much the main characters' personalities changed over time.
My mc when I first came up with him was the naive idiot type who's also incredibly overpowered (ya know that cliche) but now he's a cold and blunt guy who tries to get everyone to pick a fight with him so he has a reason to beat the shit out of them. And the other main is a talking sword who used to be the loyal and stern-ish type but then he somehow turned into a snarky bastard.(which I think I failed to portray because he kinda just turned into a regular old asshole.)

Hm, that's true... I guess only really strong/particularly bizarre or entertaining changes would be of interest to people who haven't/can't read the source material?

Any I have of that type are part of my games with my friend, though, so they're not "changes nobody saw".