This is all Good Good Stuff. I hadn’t thought about letting the players add to the world. I have a fairly defined world I want to use, but it’s got plenty of space for that. I’m just not sure if my players would be up for it - worldbuilding is maybe not their forte. I’d definitely talk out the basic idea of their heroic destinies (if any) beforehand though, so they get something they’ll really be into. Special powers and making them ’actionable’ is something I vaguely thought about, definitely a good good idea.
As my plans are, the party would be handed their quest (to stop an evil god from ascending into the source of all life itself, without going into detail) by a monk who gets killed while staying the night in their village (mysterious stranger!).
Said monk doesn’t actually die, though, thanks to some monk bullshit. His decapitated head will insist on them taking him to the sanctuary of his order so he can warn them about the evil plot. As this likely won’t be enough to motivate the non-heroic types, he’ll claim the fiend that murdered him will realize he’s not dead in time and return to kill all of them, too, since they know about the plot now. Everyone they care about too, if a more emotional angle is needed. They will only be safe in the sanctuary. Whether this is a lie or not depends on whatever proves narratively most satisfying.
There’s three things I want to do with this. First, he can gently prod the players into the plot’s direction if they get too sidetracked. Second, he can give advice if they get stuck. Third, comic relief - he’s a cranky old bastard due to the being murdered thing. By having him just be, well, a head, there’s no fear of him upstaging the players or becoming a crutch, but I can still give some in-world direction with him.
He’ll be used as a projectile the moment they enter combat, but that’s only a bonus. Also bound to be fun: explaining why they’re carrying around a decapitated head.
By semi-freeform I don’t mean that I wouldn’t plan the stuff, more that I’d prepare areas with possibilities and planned adventures they might or might not choose to undertake. The overarching goal of the first ’book’ would be to get to the sanctuary, seeing and discovering the world in their journey there. From there: your basic MacGuffin hunt, probably, except the MacGuffins are people.
Each book would have its own overarching goal and theme, and each would escalate or change things - as HB said, I want them to smash the status quo, if not always in an epic or grand way. I want to invest in some long-term things here; the fiend assassin will be something terrifying to escape in the early game, until they are finally strong enough to make a stand and defeat them in the mother of all cathartic moments, for example.
Fantasy SWAT sounds like a fantastic idea for a campaign, by the way.