I'm interested in whether you think this goal clashes with "be a fan of the PCs" - or if the latter is important at all in your eyes.
It clashes. I won’t say it’s not important, but I will say it depends on the agenda. Just like being a neutral referee is not appropriate for certain styles of play, being a fan of the PCs is likewise not appropriate.
In a game like Dungeon World, my job as the GM is to provide the PCs with adversity based on what they have told me based on their bonds, etc. This is necessary because fundamentally we’re playing to learn who these characters are. If the paladin dedicates themself to slaying the Demon King, then there better be scenes framed relating to that conflict. If I just frame things neutrally, then I’m not doing my job.
Whereas in the game I’m running, my job is to provide the milieu. If the PCs want to do something, they have to go out and do it. A paladin who wants to slay the Demon King who doesn’t take steps to do that will find the status quo mostly unchanged. They’re going to have to attack that problem and figure out the path forward. Sometimes I will provide adversity, but that follows as a consequence of the paladin’s actions.
For example, if the paladin kills a local Demon Lord, that is something that should have consequences. The PCs’ actions do not happen in a vacuum. To maintain my neutrality, I need to delegate that to the system, which will decide when and if the fallout happens. I use clocks for this, and they’re also handled transparently. The players know as soon as it happens, but the PCs only know based on how the ticks manifest in the game world (and they must because otherwise I’m wasting my time simulating the world for my own sake, which is not the point of play).
I should note that while some drift is possible (there is a particular situation in my current campaign involving a vampire ally one of the characters seemingly hates, and determining whether she would pull the trigger if she had a vampire-killing gun is something I’d like to play to find out), it’s never going to be the primary agenda, and it can’t interfere with the primary one (meaning the vampire ally is going to continue per suing her agenda, and we’ll see what happens should the character ever get strong enough to go against the ally).
Hopefully that provides some clarity. I would list both PbtA and FitD games as influential (among several others) even though I’m obviously interested in a different style of play. Many of the techniques they use to good effect also work very well for what I’m trying to do. I just have to be mindful of my overall agenda and avoid doing things to undermine it.