I think this is the big thing.
For my own game, I want to reduce the power and responsibility of the DM, and increase the narrative power and responsibility of the players.
Now I know that's a break from tradition!
But here's how I would love to play out your example:
The players decide to explore the mountains.
During Session 0, we had divided up the map, and I'd asked different players to come up with descriptions and histories for different areas. So I turn to Susan and ask her to describe these mountains, since this is her territory. From her notes or imagination, she details the topography, flora, fauna, etc.
I roll a random encounter... Two trolls! I ask the players what two trolls might be up to in the mountains. They come up with the idea that they have an abandoned guard tower they've turned into a hunting lodge (hunting people, of course). I have some of the players come up with descriptions for the tower as I sketch out a map.
During combat I have the players describe their own actions and spells, and sometimes I ask the players to help me flesh out these trolls. What are they wearing? What makes them distinct?
Now at the end of the encounter, the characters have a cool tower, and I give the players time to plan out what they want to do with it, taking notes so I can provide some rules, structure, or costs for their goals.
That, to me, would be a fun session, and shouldn't "die by fire."
(Oh, also, saying someone's idea should die by fire is a pretty, um, inflammatory way to add to a conversation!)