Well, I have a little theory that Hoid is a Sliver. (Definition check: someone who held the power of a Shard and lost it, like Rashek and Vin with the Well) This comes from the description of Slivers being like deflated balloons. In a Realmatic sense, I take that to mean a Sliver's Spiritweb has cracked so wide as to allow an amazing amount of Investiture in, allowing them to get more Investiture without being "filled". I use this to explain why Hoid has Lightweaving, Allomancy, Feruchemy, apparently the First Heightening, etc. He is able to get an incredible amount of Investiture because his Spiritweb is cracked enough for it.
And what would make Hoid a Sliver? Likely something that happened about the time of the Shattering, and we know Hoid isn't completely innocent in all that.