Well, that's my first trilogy. I think I improved quite a bit as I wrote these books, and hopefully this ending will satisfy my readers. The inevitable question is going to be "Will there be more Mistborn books?" The answer is "Probably." However, know a few things.
First off, the next series—if I do it—will not include Vin or Elend. They're dead. That's just the way it is. Sorry.
Sazed might make an appearance. He is God, after all. TenSoon is still around. (Sazed stuck the spikes back into him and the other kandra.) Marsh may or may not make an appearance. (I haven't decided if he will survive or not.)
Spook, Ham, and Breeze probably won't make an appearance, though, as I would plan to write the next series some five hundred years after the events in this trilogy. (Remember, TenSoon—as a kandra—is immortal. Marsh is also functionally immortal, as he's both a Feruchemist and an Allomancer, and can combine the powers to reverse his aging. Assuming he has enough atium left from that batch he stole to keep it up for a while, and assuming he managed to grab some cover before the world ended.)
However, this won't be for some time. I've got other projects I want to do, not the least of which is Warbreaker and (probably) its sequel. After that, I want to try a longer series, maybe a five- or six-book one. [Editor's note: Brandon was referring to the Dragonsteel series, which he's now put off in favor of the Stormlight Archive, book one of which, The Way of Kings, comes out on August 31, 2010.]
We shall see.