What are some "What if...." movies?

I thought Joker kind of fits the scenario.

What if average appearing social-worker over-medicated nutcase who looks like he smells like pee on a good day turns into one of the most upsetting villains of all time.

West Side Story. What if Romeo and Juliet, but as a musical in New York?

Inception, perhaps?

What if inception was a real idea.

District 9 -

What if aliens arrived... and they were easily subjugated, made indifferent to and mistreated by people who've gotten over the shock factor of their arrival and now treat them like a burden.