Yeah, that would be a pretty good back story. I am picturing it as just another planet similar to Earth, inhabited with humans (most plants and animal species are the same or similar, and there is a single moon too LOL). Geography and technology/knowledge are different - good enough.
There are reasons however why I think it's better left unsaid/ambiguous:
- By spelling it out you gain nothing, better let reader unleash his/her own fantasy (or SF construct) and imagination. (Both SF-inclined and fantasy-oriented people can have their own back-story).
- By providing it, ASOIF would officially become SF-Fantasy, and that would endlessly confuse distributors, book stores, marketing and licensing people etc., they like clear-cut labels.
- Even though George's SF novels are as high in quality as ASOIF, he never had much commercial success with them, while ASOIF is a smash-hit, so I guess he doesn't want to jinx himself.
- By leaving it as Fantasy-fantasy you do not have to provide a scientific explanation for every little thing, can be distracting (and conflicting down the road).
I love the idea of human-only aliens, who developed different technology, have their own history (which parallels in many ways Earth history) etc. It was not explored in SF all that much, I mean we almost have alien-archetypes in popular fiction:
- Unimaginable monsters (Alien).
- Exact opposite from the above - Utter innocence (IT, Avatar etc.)
- More humanoid aliens (That stereotypical alien with huge head and black eyes, Engineers from Alien/Prometheus etc.)
[There was a comic-book fantasy series (by Rosinsky/Van Hamme, the original run) - Thorgal, that executed the idea of human-aliens (and Earth-human visitors, similar to what you described) pretty well. It starts off with viking stories, transitions into Lord of the Ring type fantasy-adventure (with magic toned down to a minimum) and 2 or 3 major arcs are SF-fantasy.]
Few interesting things about the technology in ASoIaF:
- There is quite spectacular heating system in Winterfell, would love to have something like that. Granted, Romans did build bathrooms atop natural heat sources, but entire Stark's heat-distribution system is well above and beyond. At the present day we are still using external contraptions and radiators (heating embedded in walls would be quite expansive I guess).
- Food and gastronomy are not of this Earth LOL.
- Realm (as united by Thargerians) is the Roman empire of "Planetos", "dark-age" of course came afterwards on Earth (dark-age in many ways was a result of the fall of mighty Roman empire). It's reminiscent of Harold Foster's wonderful Prince Valiant series, a romanticized version of history, mixed up with English folklore and mythology. Camelot is pretty much a Rome of this alternative history; while banners, armory, clothing and of course tournaments George draws from there (more so than from actual history), and it's good that he does - I love it. It's not a homage (too long for that and embedded everywhere), and it's not a rip-off (story and characters are original), rather the atmosphere and look and feel - he nailed it. It's interesting to have fiction derived from fiction (Prince Valiant) which was in turn derived from fiction (romanticized history and folklore + myhtology), whilst at the same time human characters are sport on - hard ultra-reality.
- Solars gained in popularity in late 15th century in northern Europe, while they are widely used in castles all over Westeros.
- Fabric and clothing, they are using linen and silk. It was Marko Polo who brought silk to Europe way after middle-ages. Colors too - vivid colors and higher palette would probably shock a commoner from Euro middle ages.
- Tecnological progress is way, way slower on Planetos; that may not be by design but gels perfectly nevertheless.
There are probably many other examples of different/asynchronous technological development on Planetos, it's just wonderful read altogether, I am only half-way book 1 (1st read).