Would like to see, Carl get educated by Nat and Ethan in a threesome.
As stated a couple of posts above, the path where Carl begins to notice that his feelings towards the girl in frat party have disappeared and he has that 300 heartbeat when Nat is nearby, yet tries to wash the feelings away because...well, because he's nerd and a decent guy and she's married.
Nat, being a woman with experience, starts noticing the shifts in Carl, who's no longer talking about that girl yet somehow starts to be extra dorky while she is near. Pretty soon she starts to realise that whenever Carl compliments her in his own dorky way, she doesn't take that as a compliment from an acquintance whom she has platonic "mentor-apprentice"(non sexual, only talking at best) relationship, but as a young girl being praised by a dude she likes - butterflies in her stomach, etc. She sees a younger version of Ethan in Carl - smart and cunning, with lots of potential being hidden in shell of shyness and low self-esteem.
With Ethan turning into another corporate worm, who only cares about raising stocks and cutting deals, she starts losing faith in her husband, who barely pays attention to her.
Both Carl and Natalia deny the feelings they have towards each other. But at some point, the young can no longer hold it within himself and spits it all in her face. Natalia's still in denial and decides to ghost the kid, but ends up calling him. What goes further is up to your fantasies.
P.S. Yup, willing to have a love story in routes different to vanilla. Could've been some kind of coworker from Nat's office earlier, but that option is gone at this point. And none of the other characters would fit the "Love Story with cheating" trope.