The idea of a hypothetical-invention tag seems to be gaining mild acceptance:
https://space.meta.stackexchange.com/a/1167/58
If we're going to have this tag, we should probably first establish what guidelines questions of this type will need to follow, to mitigate the proliferation of low quality questions. Once we settle on some standards here, we can add them to the tag wiki.
So, feel free to jump in with any suggestions below.