^ yeah of course you'd say that, Vincent lol! Your saying that is what has me looking for it in my own play.
Here's a scene. I'm the MC.
Chopper: [rolls the harm move] I got 7.
MC (me): Okay, so yeah, while you and your gang are fighting the advance raiding party, you collide with one of the buzzard buggies. You go flying, and maybe black out for a second? When you get your wits about you, you're on the ground in the middle of the battle, pinned under your bike. Like, it's laying in such a way that you're not crushed under it or anything, just pinned. What do you do?
Battlebabe: Oh, wouldn't it be cool if right now your dead wife came to you in a vision again?
MC (me): Eh... I dunno.
The rest of the table: Oh hell yeah! She should show up and give him a raw deal to get him out of this jam.
MC (me): Eh... okay. So yeah, from where you're at it's all chaos and smoke and diesel and, you know, fumes. And you know how the psychic maelstrom is always scratching at the corner of your senses? Well, I guess it's going to rip open your brain right now. So roll that...?
Ignore for now if my calls as MC were doing it right or whatever.
Assent v authority:
I'm the MC, the rules imply I'm supposed to be saying 'what happens next' or whatever, and a traditional RPG would even make that more explicit. But in this case, the players have pitched an idea. They want to see another scene where the chopper's dead wife shows up, and they want her to give him a hard bargain of some kind in exchange for helping him in whatever way. We've already established she's a malevolent agent of some kind in the maelstrom, but we haven't gone much further than that. I'm not particularly keen on seeing that crop up at this time, but the table is pretty in love with it, so I went with it. It turned out to be great. Actually, it turned out that once I looked at how things had been going, the chopper's dead wife had a lot to be restless about -- in particular, she felt wronged by two other PCs! So the hard bargain wrote itself once I thought about it for a minute. And we would have totally missed this very awesome development if I had, you know, focused on 'no I'm the one who gets to come up with that kind of thing' or whatever.
Oh, here's the real timeline for this one: the chopper player gets harmed, rolls the harm move, and loses his footing (maybe as MC I went overboard there, whatever). The table decides his dead wife will appear in the maelstrom and give him a raw deal. I decide this will happen as a result of him opening his brain, in addition to whatever the results of that move dictate. Then he opens his brain and we retroactively map what we've decided (the dead wife showing up) onto the results of that roll. Then I come up with the particulars of the raw deal. So, a little backwards compared to my preferred timeline in the OP. Weird.
Here's another scene, from maybe the first RPG I ever played. I'm the DM.
DM (me): So your people live underground. You're in your cave, going through the private, ceremonial cleansing that you all do when you're preparing for your rite of passage to be a member of the hunting class.
Player: what? No, my people don't live underground! We're cat people! We live in trees.
DM (me): Uh, I'm the DM. I get to say what the world is like, and that includes the cat people. You get to say what you do.
[some arguing ensues]
GM: Fine; your people live in these massive trees...
In this second example, two things. First, I had never played an RPG before, and here I am going to jump in and run one. So I was inexperienced. Second, the D&D community and the rule books were pretty big on rule zero -- whatever the DM says, goes. I might have even quoted that in my protest to this player. I did relent to this player's protest, but not without causing needless arguing and social upset first.
So, takeaways maybe? I guess the question in all this rambling is: what's a good, streamlined rule that preserves the player and MC responsibilities as we traditionally think of them while also encouraging players and MCs to run with a) MC-ish contributions from players and b) player-ish contributions from the MC (or from other players)? The unspoken rule in play nowadays when I run a game is: if the table pitches something that encroaches on my responsibilities, don't casually dismiss it. Instead, embrace their contribution and try to find something in it that's interesting to me. It's not exactly be a fan of the PCs... it more like be a fan of the table's ideas.