idk miyabi. That would seem a stretch for me. At this point I don't want any new magic system, I mean there is only one book left and not much time to cover every thing. If you throw another system into the mix I think it might have a negative effect. I don't see Brandon doing that so late in the game.
I'm not saying your wrong I'm just saying that I feel it unlikely.
Well, reading all annotations, it is very clear that Hemalurgy is it's own magic system. It can't be just Allomancy, because there's just too much going on. Everything about the Inquisitors is based upon Hemalurgy. Allomancy and Feruchemy cannot explain how gigantic spikes inside the body wouldn't kill you; they do nothing to explain how Inquisitors are made (it does not explain anything of the process); it does not explain how when Zane cuts himself, it makes the voice of Ruin weaker in his head; it does not even explain how Ruin's voice could affect anyone with metallic spikes in them.
In fact, reading the annotations, MB3 is the book which explains everything. It should explain how the mists operate, the duality between Ruin and Preservation operates, why there is ash in the sky, and how the Inquisitors work. Knowing these things are absolutely vital to solve the problem of defeating Ruin. The book is not like MB2--it does not focus nearly as much on warfare. MB3 explains things. ...That's why we are theorizing, right? Because we want to know what happens in MB3.
I completely think there is plenty of space to explain all of this stuff. That's what MB3 is about.
Of course, if you can give me an explanation about the stuff we attribute to Hemalurgy--for example, everything happens when you have metal piercing your body--with Allomancy and Feruchemy, then I'll be all ears. But, Brandon's continually mentioned Hemalurgy as a third magic system, so I have complete faith that it shall be described in this book.
EDIT: VegasDev's quote isn't the only quote referencing Hemalurgy. It happens a lot, really...
