My collective theory on the story of HoA!
-Note : I'm not going to clutter this post with evidence. It's there, just look for it in the books and on the threads. Siting all of my evidence would make this post WAY too busy and long.-
1 - Allomancy - I feel that it will be downplayed a bit, but will still retain some of it's importance. It will still be key to defeating Ruin and the deepness, though it will take a back seat to Hemalurgy. Obviously Atium will not be as much an issue as it was in the previous books. I also don't feel there is much more to learn about it, other than minor stuff to solidify the more blurred edges hanging around from the last two books. I believe that when TLR took the power from the well he created Allomancy in order to attain his immortality and to control the people through rewarding them with it.
2 - Feruchemy - Where we learned a lot in WoA I believe we will get what is left from HoA. I do believe that the metals we have not heard about yet will come into play, but probably only minorly. I do think that there MIGHT be a possibility of them posing a semi-key plot. -see Sazed- I think that Feruchemy was created by a past claimer of the power from the Well and it was created in order to preserve thing, thus linking it to Preservation.
3 - Hemalurgy - We will get the rest of what is involved to creating a Hemalurgist. I believe that we will find it will concert the following. :Sacrifice(s), What metals do what: I also think we will find that those with either Allomancy or Feruchemy gain great advantages in those if they attain Hemalurgic abilities. I think that this was probably the first of the powers to have existed, thus it better binds the two others and ties it all to the mists.
4 - The Mist - Vin's theory about it being the Deepness will be proven. We will also learn how it is she burns the mist. I personally believe that it is a combination of using multiple systems at the same time creates the ability to burn the mists. I believe that using all three magic systems and burning the mists at the same time will be some ultimate power that has yet to be discovered and will be important. After all, look what Vin did with just two.
5 - Ruin - Ruin needs a physical form in order to affect the physical plain. That is the thing that it needs. I believe that the inquisitors will somehow provide this gateway that is required. How it will happen? Not sure yet. I also think that Ruin is heavily tied into Hemalurgy.
6 - Preservation - Preservation has been buried within one of our protagonists. Who I'm not sure. I think it will either be Vin or Sazed though. Also I think preservation is heavily tied to Feruchemy. I think that Preservation planted itself into a person so it could affect the physical world in the same way that Ruin needs to find a gateway.
7 - Elend - Elend, now having Allomantic powers, will be able to feel more a part of the team and will be more accepted by the remaining crew members. He will continue to play the same type of character in the third book. I think he will be the one to figure out, but not actually cause the "unity" :read that at the end of this post:
8 - Vin - She will again discover more about herself and her powers. Her having access to Hemalurgy will be heavily relied upon as they learn how it is used. -There is no basis other than my theorizing on this next part.- She will discover she also has Feruchumical abilities which will lead to her being able to unify the powers.
9 - Sazed - He represents more than knowledge and preservation. He represents all of humanity in a struggle between logic and belief. He will uncover the rest of the Terris religion and he will learn to believe. . . not in a religion, but balance.
10 - The Unification - I believe that the only way to defeat Ruin will be to unify Ruin and Preservation into a single entity. This will, in my theorization, happen when Vin uses all three magic systems together to call upon the mists. Unifying the two magics that represent each extreme with Allomancy, the middle ground together, causing balance in the world.
So it's almost 3 A.M. here. . . haha. I just thought this would be a good way to put everything together.