All I'm saying is that the fact that Unicorns exist doesn't imply that every single book is correct, and more importantly that we shouldn't let a handful of books that represent the perceptions of a few wizards prevent us from inventing a story that is entertaining.
That said, I feel that at this point inventing a totally new universe is far better than shoehorning space travel into TES.
Imagine, you could have a cosmos built around a series of moons surrounding a gas giant. They could each represent a nation of sorts, giving our setting an assortment of magic and technology that leans to the space opera or even planetary romance side of the scale.
How about this: A humanlike species evolves on the earthlike moon of a gas giant. This moon is smaller than earth but denser, like all the moons of this giant, and thus has similar gravity but less surface area. This race eventually uses technology we might distantly recognize to travel between these moons and terraform them to a variety of climates; there are dozens, and they fit a variety of niches (deserts, oceans, forests, ice, swamp, fungus, crystal, etc...). All sorts of techno-gadgets exist, from guns to teleporters to spaceships to AI.
A cataclysm not only stops all contact between these planets, but also blasts every world back to the stone age; this could be the sudden appearance of magical forces or the failure of the main sources of power used by the interlunar society. Perhaps some kind of doomsday device used in war, although this would presumably lead to a certain amount of fallout, which might confuse the setting.
Over the course of, oh, ten or twenty thousand years, the various now-isolated remnants rebuild. By the time they contact one another again, they have forgotten all of the history they once had and have even divergently evolved into nearly separate species. At first, contact is solely by semaphore- flashing sunlight off large mirrors- but after many, many years, the secrets of the ancients are being recovered. Many of them are lost forever, and because many of these secrets are being rediscovered, technological progress is not linear. Archers and swordsmen board ships capable of traversing the void, and mages question demons as to the best orbital trajectory for a flight around the central giant.
Our story could be set decades after the first voidships are built, or it could begin a century after first contact, now that the various groups are intermingling, and we might find anything from a majestic port city where people from all cultures come to trade to a hidden void pirate moonlet to the central temple of a icy world's xenophobic religion, containing the manifestation of their god, the holographic avatar of a malfunctioning defense computer.
Naturally, one of these planets could have plentiful natural caverns and low ceilings, and mushrooms just bursting with brewable carbohydrates. That's only to be expected.