This Is Us constantly flirts with science fiction/fantasy tropes of time travel without ever touching them. For instance, Season 4 opened with a flash-forward to roughly 2050. In Season 4 Episode 17, This Is Us seems to jump multiverses, showing a very different Pearson family, one where Jack survived the fire. It left fans asking so many questions about Kevin and Kate's future spouses, including who Ethan, Kate's alternate-reality husband, is on This Is Us. Sorry, Toby, you're out of luck. Warning: Spoilers for This Is Us Season 4 follow.
As noted above, This Is Us is not in the habit of playing with time and space in this manner, which makes the two alternate timelines where Jack survived something of an oddity. More than anything time-bending, they are fantasies inside Randall's head.
The first fantasy Randall conjures is where all the good things in his life happen, and all the bad get fixed. He still marries Beth, except now Jack makes a toast at their wedding. He and William reconnect much earlier, and Randall catches that William has stomach cancer before it becomes terminal. Jack being alive means that Rebecca's memory issues get noticed quicker. All the joy, none of the pain.
That's why the second fantasy is far more interesting. What if instead of connecting with William at 17, Randall was rejected by him? What if Randall couldn't forgive Rebecca for not telling him the truth? Suddenly, Randall never meets Beth. Instead, Kevin and Kate are the ones who wind up marrying in their 20s, with successful careers and happy lives.
In this fantasy, Kevin and Randall switch places. Randall is the family screw-up, Kevin gets married to Sophie, and Jack makes the toast at their wedding instead. Kevin doesn't become an actor; he joins Jack's architect business, "Big Three," aiming to inherit the company one day.
But it's Kate's life that Randall creates that's a little shocking. She's a bubbly party girl, planning big July 4th blowouts at the cabin. (It's not unlike her personality when they were little kids.) She's also married with twin daughters. Her husband, Ethan, spotted just for a moment in the other room, is really hot.
Where did this guy come from? Did Randall's brain make him out of whole cloth? It seems likely. Kate doesn't have a happy significant relationship for Randall to pull from, unlike Kevin and Sophie. In real life, she dated Marc, who was abusive and terrible. The name might be the writers referring to that — Marc was played by Austin Abrams, who plays Ethan in HBO's Euphoria. But that's a bit of a stretch.
Either way, fans are curious about what other versions of Kevin and Kate's lives Randall might have cooked up.