Architect student here. :wave:
Like most of EPCOT Center's original Future World architecture, the design of the Horizons building is delightfully modernist and abstract. Therefore, it's unlikely that the Imagineers were attempting to imitate any one object in particular... ie, a coffin or spaceship

) Occasionally architects become so absorbed in designs as they change and evolve, and become used to what a building looks like at an abstract level, that we lose sight of what our building may look like to a person who sees it for the first time, and wants to see it for something more literally.
That said, here's my take: if you look at the elevation of the building, you'll notice that the vertical/diagonal all would convene at a single, invisible point:
In drawing, we would say that this point is on a
horizon line - probably not a coincidence in relationship between the form of the building and the name of the attraction. Here's a diagram of a one-point perspective, and even in this simple diagram I think you can see a similarity in concept to the form of the Horizons showbuilding:
This, then, would suggest that the building and its ridges are pointing or aligned to a distant, far-off point - a
horizon. :animwink: It may also be implied that building is approaching or climbing toward this horizon, in the same way that humans are reaching for their future through progress and innovation.
This is also illustrated in the attraction's logo:
But wait, there's more. The sloped walls and lines of the Horizons building have the added effect of causing the viewer to look skyward, subconciously encouraging one to think about the sky, space, or the future (Space Mountain also masterfully uses this effect). Check out that Horizons attraction poster again, and notice what part of the image is given the most size and attention - the sky.
Finally, I think there is something to be said in the fact that the horizon line is not depicted, but only suggested architecturally, in that the vertical lines never actually convene, and the form of the building stops abruptly at a horizontal line. This may likely represent that the future of mankind is unfinished, and is still being built today.
Like I said, abstract stuff. I hope that cleared things up a little.