SuperFly

IF Mechanic: Knowledge gating-

Let's say you have a story centered around a highschooler and their favorite uncle, who is a bit of a crackpot scientist; sort of a Marty McFly and Doc Brown dynamic. The kid usually spends time in his uncle's barn before and after school helping him with whatever hairbrained scheme he has in mind. The morning was nothing special, with the kid promising to stop in and see his uncle later that day because he's going to have something to show him that will knock his socks off. While at school, he goes about his day as usual. However, when he returns after school, the barn door is locked.

After repeated knocking, his uncle findly answers through the locked door.

"What do you want?!"

"It's Paul!"

"Paul who??"

"Your nephew, Paul!"

"...I have a nephew? ...Go away!"

"But..."

It was clear that he had already walked away from the door.

To promptly spoil this set-up, it may be his uncle, but it isn't HIS uncle.

HIS uncle had been secretly studying reflections. He had discovered that reflections in mirrors are actually real time images of nearly identical alternate universes that only have a single tiny, typically trivial, difference. Like everything is the same, except a random guy named Pierre from Quebec wore a blue shirt instead of a red one when he turned 12 years old back in 1983. Everything before and after is the same. Each reflection is a view into another different near peer universe, it too with it's tiny slight imperceptible difference.

His uncle had discovered that these reflection universes synced based on the frequency of these worlds being a near match. Knowing this, he tried to cover the mirror in a thin coat of different substances in an effort to alter how our universe's frequency propogated through the mirror. He figured if he succeeded, the frequencies wouldn't match well, and maybe the image in the mirror wouldn't match anymore.

.

What he didn't consider is that he'd be altering the reflection seen from the other side as well. He also didn't figure that, using a large mirror in his barn, he'd be looking into the workshop of a version of himself who had made all the same discoveries, or perhaps even more... like how to cross the reflection into the other universe.

The plot follows an escalating mystery of his uncle becoming increasingly erratic and a large draped mirror that doesn't reflect Paul, only the empty lab behind him. It isn’t until near the end that it is revealed Paul's uncle isn't Paul's uncle, and he manages to thwart the man taking his place and discover what is going on, causing his not-uncle to flee back through the mirror.

Paul realizes his real uncle is being held captive in the mirror world his not-uncle fled to. Using the special command/verb needed to enter the not-uncle's mirror universe ends the game in a cliffhanger cut-scene. Cut to credits. To be continued.

Fair enough, right?

Now imagine replaying the game later and noticing the school section near the beginning of the game has a large mirror in the boys bathroom.

On a whim, you try the command you will only learn near the end of the game, and it works, and you cross into that mirror universe to the astonishment of the Paul there.

From here, you can now skip school (very confused other Paul is still there) and go to the barn to speak to this universe's version of his uncle before he completes his experiment.

This option was always available, but the player didn't know to try it. It was wide open, but gated by the player's knowledge or lack thereof. 

Now let's say, talking to this universe's version of his uncle, he learns something critical.

Well, this critical topic was always a topic option with the uncle before school, but the player didn't know to ask about it.

So, on the next replay, if this topic is brought up, "ask uncle about X," it flabberghasts the uncle, who asks how could you possibly know that. At which point, you can tell him you have been stuck in a timeloop, reliving the same day over and over. This revelation completely changes the course of the game and this playthrough bears no relation to the first two.

All of these options were there upon your first playthrough, no game states are changing to open new paths for a newgame+.

They were only gated by the player's knowledge. Ostensibly, you could have the same game double back on itself a dozen or more times with no indication that the initial playthrough isn't all there is to see, since it already has a logical and coherent ending as is.