The beauty of the Fractal of the Mists dungeons is that they allow ArenaNet to push out wacky and off-the-wall ideas which they might not necessarily be able to knit together into a full dungeon - but are nonetheless fun enough to work as a 20-30 minute instance. Fractals support ArenaNet's half-baked ideas (and ANet are the kings of half-baked ideas - hello "personality system") and give them leave to do... well, pretty much whatever they like.
The current Fractals are a mash-up of lore-based historical events, trendy pop-culture references and funky new mechanics. The Cliffside Fractal (also known as the Colossus Fractal) obviously came from someone saying: "Wouldn't it be cool if we had a dungeon with a gigantic living statue?" - this idea was probably slammed together with "I've got this idea for a mechanic with a powerful weapon which hurts you the longer you carry it." and mixed with a bit of Dr Manhattan from Watchmen. And out of this mash-up of crazy and interesting ideas pops a really fun little vignette (one of which I've yet to find a single detractor). Some Fractals might have been inspired by something as simple as a piece of concept art which never got realised in-game.
Also, because the Fractals occur "in the Mists" rather than in the tangible world of Tyria, they aren't truly bound by lore, space or by time. Sure, they're often significantly influenced by these factors - such as the "Urban Battlegrounds" Fractal which seems to take place during the Searing of Ascalon. But others seem to have little basis in anything we've seen before - such as the "Aquatic Ruins" or "Snowblind" Fractals. There is no need for ANet to justify where or when these events are happening, because they're occurring in a vacuum - a totally separate universe to the rest of the game world. The Mists is a true "play space"; unbound by many of the rules which ANet would have to abide by if they tried to play these stories out in the wider world.
So, they have the freedom to mess about, play and mix up just about anything they like. Imagine a Fractal which is a mash-up of Hogwarts (moving staircases, hidden doors behind paintings, ghosts on horseback riding through the halls*) and the historical event of the Foefire! Or festival themed Fractals such as a Dwayna vs Grenth battle for Wintersday (the withdrawal of the Gods from Tyria wouldn't matter in this instance, it's a Fractal, after all!)! Or, even, imagine they took the swimming animations and just stripped away the water and then stuck you in some ethereal limbo world or even accompanying Snaff through Kralkatorrik's mind.
I dunno, I'm just throwing these out there (though if any of these ideas appear in the next patch, I'll be expecting some sort of recompense). My point is that if they're going to use Fractals to let their imaginations run wild, then I think we can expect some pretty freaking awesome experiences in the future.
*this always confused me. You've got the Necrotic Horsemen from GW1, but I never saw a single horse in the entire game.