I will add my own thoughts.
I very much believe in the expansion through play rather than hashing out everything beforehand. ((says the guy who spent 4 years working on Graterras, though that was long form rough idea forming.)) Hell, a lot of my ideas came from it alone. ((Such as the dream sequences in Chapter 3 of World in Peril, the RP I am running for my players..)) While, I did build some of the world and lay out the structure of the way it would be. I knew there would be 9 islands and basic stuff about the homelands of the races etc... However, I never ever drew a map. I didn't plot in the fine details. Those are what I would use the game to pencil in. I would visit those places along with my players and see them with them. I would get a feel fro my world as they did and be just as surprised as them in the wanderlust.
In Story, I strictly believe in what Adam and Steven will expand upon in later episodes. ((cause I watched all of them back to back they are lumping together for me XD>)) But the story/plot of the RP simply doesn't exist yet. It only exists after the players have gone through etc... While I have a direction I would like to go, I am flexible. I am willing to change that plan on the fly and in reaction to my players and what they as characters do. The Death Knight fight in Chapter 2? I Actually was pretty sure I would kill at least a character. However, the brilliant combo that Akuma and Sigurd set up along with Odin impressed me enough that I decided That the Death Knight would fall much, much sooner than I planned. ((Though Brass's sacrifice definitely weakened him a lot too.)).
And on your point of that a Host should listen, in a later episode they will talk about the Rule of "Yes, and." It is too take such a suggestion of a player etc, and expand upon that suggestion in some way. This rule is taken directly from Improv Theatrics. ((and a lot from Theatre is actually helpful in RP.)), other variations of this rule Include "Yes, but" in that it is slightly different than normally suggested and "No, but". This is a technique to force yourself to think of things or to adapt Player suggestions to something that would work in your world. Now, don't do it all the time. SOmetimes you just shouldn't include a suggestion because it is asked. Use your discretion. If you think it wouldn't be good fro the world or the RP explain this to that player.
In short, I like the more only finely prep what your player's will be touching in the near future approach. It prevents you from overloading yourself and leaves you room for variation and to change up the story as you go along. It also allows you the Host to be surprised by your own world sometimes, which is just as fun and excited fro you as it is the players.