Super Doppelganger / Double Doppelganger magic. Stefan is a doppelganger. Katherine is a doppelganger. Katherine turned Stefan into a vampire. Stefan has a double dose of doppelganger magic. Stefan was born a human doppelganger and was turned into a vampire by Katherine’s doppelganger blood.
Doppelganger blood has no effect on humans, but what about on another doppelganger? It could be argued that since Katherine is a vampire, her doppelganger blood is useless and therefore shouldn’t have had an effect on Stefan, even if he was a doppelganger. However, TVD has never clarified whether it’s only HUMAN doppelganger blood that is powerful and useful or if given special circumstances, VAMPIRE doppelganger blood may have some kind of magical properties. Klaus needed Elena’s human doppelganger blood because Esther used Tatia’s human blood for her spell.
You’re either a witch or a vampire. Never both. Stefan is from a witch bloodline, born a doppelganger and he was turned into a vampire by doppelganger blood, which is very powerful according to Esther. Maybe the combination of these factors could have turned Stefan into something more than a vampire, like part witch.
If Stefan was a Witch/Vampire Hybrid, we would have seen Stefan display witch powers. Maybe Stefan’s witch powers work a different way or their dormant, or he has witch powers but not enough to cast spells like Silas. Silas already exhibits powers exceeding that of a Vampire – he could be the first Witch/Vampire hybrid – having some witch powers but not enough witch powers to cast spells. TVD hasn’t clarified what Silas is EXACTLY – a immortal, a vampire or a witch/vampire hybrid?
Is Silas's Doppelganger Bloodline more powerful than the Petrova Doppelganger Bloodline? Silas's doppelganger bloodline is older than the Petrova doppelganger bloodline by at least 1,000 years. In addition to that, Silas's doppelganger bloodline is also a WITCH bloodline.
Nature may still be in balance even with Stefan being a vampire since Stefan can still be killed by burning up in the sun or by a wooden stake. Since Stefan didn't have any children before he became a vampire that could also make him Silas's last Doppelganger. So, if Stefan is killed by the sun/wooden stake - and Silas is still alive - how will nature balance itself out with no doppelgangers available? Will nature resurrect Stefan over and over again making him sort of an Original Vampire, but not quite? Stefan could be killed by the sun/stake - and then comes back to life - thus fulfilling nature's death clause required to counter Silas's immortality and since Stefan is the only and last doppelganger - nature would have to bring him back to life to keep dying for Silas's immortality.
Stefan died once as a human in order to become a vampire - thus serving nature's purpose. But as a vampire, if he is killed a second time by the sun/stake - that's like a double death. Is Stefan balancing nature and unbalancing nature at the same time by being a vampire doppelganger and Silas’s last doppelganger?
Did someone send Katherine to Mystic Falls to turn Stefan into a vampire all those years ago? After all, Emily Bennett told Stefan that Katherine had compelled Stefan to drink her blood - FOR WEEKS. That doesn’t seem the normal procedure, it’s usually pretty quick – feed the human the blood and then kill the human as soon as possible. Unless, Katherine didn’t want to take a chance and needed to make sure Stefan became a vampire no matter what. This may support the theory that someone knew that Katherine’s doppelganger blood – might turn Stefan into a different kind of vampire since Stefan was also a doppelganger.
Since Stefan and Elena are both doppelgangers - will this become similar to Damon/Elena's Sire Bond? Is what Stefan and Elena feel for each other real - or is it because they're both doppelgangers?
I’m sorry this is all over this place. I hope some of this makes sense and someone understood my chaotic thinking. Yeah, some are weak theories/arguments and have a lot of holes in them, but I thought I would throw it all out there.