I've run two successful one shots with different degrees of success. The first I completely improvised, no notes or anything, and the ending did feel a bit rushed but we wrapped it up in one session. The second I ran with just one major theme in mind and the rest was improvisation. It helped that I set them up in-universe with the main campaign so the end goal was just to create consequences for the main party, good or bad.
I love the Mass Effect series! (Only played the remaster) but I loved the way my choices had consequences throughout the trilogy. Like the dialogue trees were really fleshed out for their time, I'd even argue for modern day.
Isn't this the plot of the first Cloverfield movie?