A thought (by a dumb reader :P)

[Tglassy]

Hoid is definitely an Allomancer.  In Stormlight, he's seen dumping metals into his tea during one of Shalan's flashbacks.  

Rashak became a Fullborn, I believe, because he was already a Feruchemist, and then he ate one of the Lerasium Beads. Possibly he just gave himself Allomancy, but so much more cost efficient to just eat one of the beads.  There's a way to use Feruchemy to strengthen Allomancy, I think by tapping Investiture or maybe Connection, and since he could Compound every attribute, he could compound whatever he needed to to increase his Allomancy to insane levels.  

Also, to the original post, Allomancy is of Preservation, Hemalurgy is of Ruin, but Feruchemy is of both.

[Raphaborn]

On 10/12/2021 at 9:28 AM, Rashek's PR Manager said:

Even if a natural Fullborn is impossible, twinborns still pose too great a threat for Rashek, I think. Just imagine what damage Miles could've done in the Final Empire. Besides, it'd expose Rashek as a fraud.

Also, a Mistborn Ferring or a Full Feruchemis Misting is somewhat more possible.
"Very improbable" is still a very dangerous possibility when talking about 1000 years and a hundred million people.

[Tglassy]

If I'm not mistaken, you can't get a naturally born Fullborn.  The place in the Spiritweb that denotes if you're a Mistborn or Feruchemist is in the same spot, so you can't be born with both.  The overlap isn't exact, though, so you can be born a Misting/Ferring, or Twinborn.  Just not a full Feruchemist and Mistborn.  The only way I know of to do that is to be a full Feruchemist and then eat a Laresium bead.  Or use Hemalurgy.  

[Going_North_cal]

3 hours ago, Tglassy said:

If I'm not mistaken, you can't get a naturally born Fullborn.  The place in the Spiritweb that denotes if you're a Mistborn or Feruchemist is in the same spot, so you can't be born with both.  The overlap isn't exact, though, so you can be born a Misting/Ferring, or Twinborn.  Just not a full Feruchemist and Mistborn.  The only way I know of to do that is to be a full Feruchemist and then eat a Laresium bead.  Or use Hemalurgy.  

This is what I've heard. Although, the word 'natural' can mean different things. Natural for me, would be by means you cannot control. As such, by my standards, a Fullborn would be possible naturally if Harmony just... turned you into a Fullborn. For me, that's natural. And He could do it at birth, thus making you a naturally born Fullborn.

Edited December 13, 2021 by Theldragor
Added some info

[Raphaborn]

17 hours ago, Tglassy said:

If I'm not mistaken, you can't get a naturally born Fullborn.  The place in the Spiritweb that denotes if you're a Mistborn or Feruchemist is in the same spot, so you can't be born with both.  The overlap isn't exact, though, so you can be born a Misting/Ferring, or Twinborn.  Just not a full Feruchemist and Mistborn.  The only way I know of to do that is to be a full Feruchemist and then eat a Laresium bead.  Or use Hemalurgy.  

You can be a natural Fullborn, that's just too hard. And Hemalurgy doesn't really make you a Fullborn.

[cometaryorbit]

I tend to think it has to be at least remotely possible, else TLR wouldn't have worried about it. A one in a billion chance would be meaningful, as TLR apparently expected (at least sometimes - he wasn't always the sanest) to live and rule forever. But if it was flat out "physically" impossible (well, magic-physics) not so much,  as TLR had a very good understanding of the Metallic Arts.

Now maybe he feared a double gold Twinborn would reveal too much about his immortality ... OTOH he kept a lot of other powers that existed & were used secret*, why not that one? Gold Mistings were already not supposed to exist (presumably to cover up the existence of Atium Mistings). We know that Inquisitors knew & used Aluminum from book 1, and that was also secret.