It's a good idea. I believe it has been floated before in some fashion, somewhere in the boards.
Consider this then: Black Court Vampires CANNOT cross a threshold uninvited, full stop. I suspect this is to do with how far removed they are from mortals, and more likely, the source of their creation (Drakul, via Dracula).
But yes, the analogy that creation is our home and not the Outsiders, and therefore that is why all mortal dwellings are protected in that way against the Supernatural.
However, as a counter - Bob states in one of the earlier books (It might have been Blood Rites but I am uncertain) that one of the few beings that could flat stick cross a threshold and still have enough power to torture someone with magic is an Old One, or a "hoary old god". I suspect upper level Outsiders (even the Walkers perhaps) could do so too, considering the fact that it appears higher-tier, multi-reality spanning beings (Angels, and especially Archangels) seem to have no issue crossing threshold or magical circles.
Which leads me to my point about magical circles also being in the same boat, to a degree, as threshold. I think it is all tied up in Choice and Free Will (the emphasis on Will is important, as that is how you make a circle actually work or any act of magic really). The Freedom is most important also, but you can have restricted or compromised Will whereas Freedom cannot be on its own, as it indicates there must be some level of Will (even in the supernatural and immortals - WOJ on that).
Exartiem, very interesting idea about how they must borrow power. I suspect its less about Power, and more about Choice. Outsiders seem to be able to provide Power to mortals and supernaturals, and some measure of Freedom to Immortals (who traditionally by their nature are limited in their Choice). The Outsiders are very similar to the Void Walkers (etc) in DnD/WoW. The curious thing with Void Walkers is they have enormous amounts of power, but are constantly losing it in when they are inside the universe. I think the Outsiders have a similar issue. When they are invited in, it give the the ability to employ their Power. They are given a level of Choice. Without it, they would be either very limited or perhaps completely unable to act and impose their Will. Which is why they can only be summoned via MORTAL magic. Immortals do not have the unlimited, truly Free Will that mortals in the Dresdenverse seem to have. Perhaps they borrow the body of the summoner, and their soul (granting access to their ability to Choose). Perhaps not, we can really only speculate on the mechanics...although He Who Walks Behind did possess (Madge? the main Witch) in Blood Rites, in order to act in the Universe. So we have partially seen how an Outsider summoning works.
This all begs the bigger question of why almost every supernatural being is somewhat hampered by thresholds and circles, etc. Where does magic really come from? What is the origin of all these supernatural beings? Perhaps it is more connected to the Outsiders than we are led to believe.
