I don't really use the threat/front system in an explicit way in my games. However, conceptually, I can provide some insight into doing so. Let's take your example.
The Landscape is the Sewers, it threatens people because it's a maze. This is basically the same as saying, there are some fuckers in a gang down the road that just shoot people that come near their watering hole. Both are avoidable in just same the way, don't go into the maze, don't go into the gangs turf. In the same way that you can drag people into the gangs turf by putting something they need past it, within it, or taking someone that the gang cares about and putting them in harms way near the pc--thus bringing the threat of the gang as a relevant story arch. You can put something the PC's need or want on the other side or somewhere in the maze, or likewise, having something dwelling the maze start to reach out and get into people's shit.
More simply, lets say you have a:
FONT called THE SLIME MONSTER. represents: Fear.
It's a monster that comes out and destroys shit with ooze and terror and whatever have you.
The dark future portrayed by this font is that the city becomes overwhelmed by the giant slime monster and dies. The stakes are asking what things survive the fall.
The Threats:
That cult that dwells within the sewers and makes freakish drugs they periodically come out to sell to people. The drugs are extremely addictive psychotics that rapidly alter behavior in strange ways Oh, and the detox kills. Brute: Cult. They fear the outsiders and worship the Ooze
Maybe the drug itself is just everywhere already. It's like a Affliction:Delusion. But we're going to attach it to being able to walk through the sewers unimpeded by the small slime, cause maybe that's where the food grows. They fear withdrawl and love the ooze.
It's led by some secret masked man that goes by many different alias', no one is quite sure who or what he is, but he is out there like some canny thieves guild master spinning some web for the enjoyment of his Ooze God.
Grotesque: Mind fucker He spreads fear/chaos and loves being "witty" or Ironic. (probably less important, I'd drop this one unless he grew up naturally on scene)
Oh and there's those addicts that are out there stealing people/supplies/things and making the trade within the sewers. This has grown to be such a biz that there's actually a warlord that has started buying and selling people to get in on the gig. Maybe he's your hardholder, or maybe he's decided to enslave most everyone and hands off the dead and lame to the cult for more taste for his guard? Who knows, he's probably out to kill the cult and take over that biz for himself too. Does the cult maintain some balance? *shrug* Warlord: Slaver. People fear him and fear what might come of the people they care about but love the existence of some half-measure of a social structure.
The Sewers that this is all taking place over is certainly another threat, Landscape:Furnace, because going in there is dangerous for all kinds of reasons, and everyone involved with this is either going into them, moving through them, or coming out of them, the slime being just one of them. The slime is probably not something that can just be gunned down for the win anyway, making the slime itself something of a landscape to be navigated. Maybe the sewer and the physical occurrences of the slime serve the same type of threat the same thing conceptually speaking. In that way, the sewers might be consuming the landscape around them, leaving less and less other then it unless it's stopped. Is that even possible? Can you so to war against a sewer? Probably. People fear the ooze, the ooze loves to consume. (altered from a Maze to a Furnace after writing cause it made more sense)
Added later for MAZE example: The sewers might just be the maze that the cult grows, where the slime resides, where the slaver trades in, and the escape slaves sneak through. Movement through the landscape is done by every member of this font, so make the Fear of what's in those sewers coincide with the groups need for what else is in them.
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The Above is haphazard, but hopefully it provides yet another demonstration of your query. I think the last maze landscape I ran in one of my games was a choice. Go into the sewers (the maze), or risk going through the furnace (above the maze). The maze was a problem because of creepy monsters, darkness, lost, and the rejects and losers from the above war. It was enticing because you could get anywhere without being seen. The furnace above on the other hand consisted of the war grounds between no less then 7 factions and a bunch of independent but brutal fuckers--pretty much everything that went in got destroyed. People, factions, the peace, the vehicles, mines, bystanders, lol. Of course, the furnace in that sitch was also where all the people lived, since it was pretty much a choice: there or the sewers.
The group was involved in the conflict, they had winners and losers. The sewers enabled them to organize different factions together and coordinate attacks. But getting lost they stumbled into what was essentially some mad scientists laboratory where he was doing genetic mutations on people and turning them into sneaky rat-men and was backed by a force of semi-sentient robots. All in the pursuit of some stranger secret that was connected in a fundamental way to the psychic maelstrom. The sewers ended up in Both fonts, but in each it did something different. In the first, it was a either going to get you where you wanted to be in good shape, or put you where you wish you weren't. And in the latter, it was more of a prison, keeping the secrets and the strange inside it, and making it very hard to get out. It was all a matter of how "deep" you went.