I'd first like to say that this series was fantastic. I was exceptionally pleased with how you tied everything together in this final book of the trilogy.
(1) This series has the best world-building, magic system, and over-arching plot of any epic fantasy I have ever read. I think George R.R. Martin is still the master of creating memorable characters, developing them, and having them interact with each other. Other authors, like Hobb and Rothfuss, are better at evincing emotion. You are an amazing writer yourself.
That being said, I have a couple suggestions for you.
(2) The first contradicts itself, so take it for what it is. I would suggest that you write how you feel the story should be written. Getting inspiration from someone is one thing, but changing your work because some people want a happy ending or dark ending takes away from the purity of writing. The part you added in at the end where Sazed let Spook know Vin and Elend were happy in the afterlife really stuck me like a thorn. I think it was apparent how happy they were together in life and how necessary their sacrifices were. That would have been enough for me.
(3) My other suggestion is more of a plea really. Please don't extend this series just to capitalize on it. If you really feel there is more story to be told, then tell it. I, for one, thought the ending would have been perfect if allomancy, hemalurgy, and feruchemy would have faded from existence as their corresponding gods did. It would have been rather romantic to have people start over with a new "normal" world.
Congratulations again on completing a masterful work!
1. You humble me. I don't think I've NEARLY the skill for characters that Mr. Martin does, and that's not just an attempt at modesty. I hope to be there some day, however.
2. This is a tricky one. I didn't change the worldbuilding or the cosmology of the story in order to fit what people wanted, but I feel strongly about using writing groups and test readers to see if my intention in a book has been achieved. I show things to alpha readers to see what is confusing or bothersome to them, then decide if that's really something I want to be confusing or bothersome.
In my mind, the presence of a powerful being such as Sazed, mixed with some direct reaching from beyond the grave by a certain crew leader, indicated that there WAS an afterlife. However, test readers didn't get it, so I tweaked the story to make it more obvious. Perhaps I should have left it as is, but I liked both ways, and decided upon the one I liked the most in the context of reader responses.
I do plan to always tell the stories from my heart, and not change them because of how I think the reactions will be. But I do think it's important to know what those reactions are ahead of time and decide if they are what I want or not.
3. We are on the same page on this one. You can read other posts on the thread to see what kind of thoughts I might have for more Mistborn books, but I don't know if/when I will write them. It depends on the story and how excited I am to tell it.