I've been playing Blades in the Dark lately, and a
D&D-esque hack of it I wrote myself. It is very open to player initiatives, and very friendly for improvisational play. Basically, every session starts with deciding what to do, so there is no way to really plan ahead, either as a GM and player. All the prep work is now about setting up the setting with plots and conflicts the player's may want to dive into. Because NPCs really only have one game rating, Tier, and the rest is in the GM's head, it is very easy to improvise.