So, spoilers all the way, then. The characters (and by extension the players) know the future of their world as well as its history. Yeah, not gonna fly.
Now if there is no preplanned future - which is, I suspect, where you're leading with this - then fine, bigger-picture events can be made up on the fly. That still doesn't mean those bigger-picture events don't and can't exist, and the guides for some of these games even say as much when referring to what happens off-screen.
Sure AW and DW both have fronts. The GM is part of the game, and I am not advocating for a lack of any input into setting, situation, or even character in some degree (though generally GMs stick to NPCs mostly). And, as I have also said, its not that there cannot be any surprises either, they simply cannot be of the type that yanks the wheel out of the player's hands. You can put LOADS of obstacles out there, heck, the more the better! They can spring up as needed. I would advise that the most substantive ones are probably anticipated, but these games surprise you! I mean, in our BitD game we killed a couple guys and took their horses. Those damned horses did 5x more damage to us than the gang that was riding them! lol. Talk about an obstacle, one horse came within an hair's breath of doing for the entire crew. It was great! Nobody can invent that stuff.
When you phrase it as "oh sorry I decided all your plans are crushed" that phrasing implies the decision was made on the spur of the moment (which I agree would be terrible form). Things like this should be laid out before play even begins and before the GM has any idea what the PCs' goals will be.
But you have to imagine, the players are not really aware of, nor terribly interested in, some list of events you might have made last week or last month. As I've said a zillion times, there's no compelling logic that demands an invasion over a diplomatic mission, or nothing at all. So in the final analysis, its only YOUR sensibility that is effected. Your satisfaction, not mine or any other player's.
And then if-when a PC comes up with a goal that by sheer bad luck is almost certainly doomed by something bigger that will happen, IMO all the GM can do is keep a straight face and carry on, even if inside thinking "this ain't gonna end well". I suspect I-as-player might have been in this very situation for some time now: my characters end goal is to overthrow the faux-Rome republic* and make it an empire, with herself as Empress; but I've a nasty sneaking hunch that nation is going to fall long before I get anywhere near a throne and I ain't entirely sure there's a thing I can do about it: I know what's out there, and I know if it comes for us we can't beat it.
No, he can, as I just suggested up thread, assuming you want to stick to your meta-plot more-or-less (and OK, you do want to do that, nobody is certainly denying that's your desire) recast it as a choice for the PC, the crown or loyalty to your country, or I'm sure you are creative enough to come up with any of 10 other ways to make a dilemma or obstacle out of this. Why should your player not have a chance of success? Why? What purpose is it serving not to give her the chance! All she has to do is marry the disgusting barbarian and she can rule (albeit sharing power). Its like you have the gold coins in your hand, but something stops you from spending them. Give it a try!
Still, she soldiers on, hoping for the best...
* - very short version of a VERY long story: when she was younger it was an Empire, but due to some cataclysm or other during the campaign the whole nation got punted 250 years back into its own history, to a time when it was still a republic. My character is still in her own "time", however, and thus has a whole bunch of memories of things in her past (and the country's history) that now may or may not happen in the "new" future.
So, none of this is a surprise to her, and she KNOWS how to change the future. Man, there are so many ways to hand the keys to the players here and make gold. I mean, D&D won't make that EASY, but you can at least get halfway there. Hey, maybe you'll invent a style of play that makes everyone happy, you can always dream!
