Then it would no longer be "like Halloween" but merely "normal"

What if you won the lottery?

Hopefully it's the money-giving kind of lottery. I'd buy every printed card on the reserved list that I could and burn all but one copy of each.

What if an extinction-level-event-causing asteroid was going to strike the Earth in 30 days, and only you knew about it?

Then the world would end because there wouldn't be much point in a nobody like me shouting "End of the World." No one would listen.

What if we were evolutionary descendants of lizards instead of primates?

~SE++

Then the evolutionists were wrong, but so were the creationists.

What if mankind never figured out how to build a structure more than 5 stories tall?

We'd be building more structures many stories deep instead.

What if humans had prehensile tails?

Then trouser design would have developed differently.

What if the year was exactly 360 days long?

Then figuring out the date would be easier and we could finally stop with all this leap year nonsense.

What if dogs were as smart as people?

"We'd still have some pretty dumb dogs....based on the people I've seen lately."

"What if God was one of us?"

He would probably play the Sims.

What if it turned out the Beatles were phonies?

Then Milli Vanilli's trick wouldn't have been nearly as impressive.

What if water didn't roll off a duck's back?

Then they would sink whenever it rained.

What if the government outlawed walking and required that people skip everywhere they went instead?

Chase scenes in movies would be much funnier.

What if cheese was never invented?

Then lactose tolerance would never have become a common trait and yogurt would not have been invented.

What if people had their exoskeletons on the outside.

Halloween costumes would be drastically different.

What if rolling low in D&D was good instead of rolling high?

Then negative modifiers would be good and lots of the math would involve negative numbers. The whole system would be flipped on its head to accommodate this change. Maybe you would count your "damage" up to a max value where you would die instead of hitpoints going down to zero. Fighting games would have evolved in a very different culture and you'd die in Street Fighter when the damage bar filled up instead of the health bar draining.

What if tabletop RPGs weren't invented until the 1980's?

Then they probably would have played them a lot on Charles in Charge.

What if mullets were considered a classy haircut style?

~SE++

IROCs would be in style again.

What if the Grinch stole Christmas.

Then everybody would realize that Christmas isn't about the presents that he took. It's about community or family or singing or something.

What if everybody walked on their hands?

Carpal tunnel would be an even bigger issue.

What if spiders could fly ?

We'd be in a SyFy channel movie. (Actually, they may have already done that one)

What if our fathers would try to kill us like male bears do to cubs they sire?