I just finished the first mistborn series and loved it! I didn’t know about the Cosmere before I started but now that I know, what should I read next? I’m leaning toward the next mistborn trilogy.
The next set of the Mistborn saga would be a natural choice. You may need a period of adjustment for the first couple of chapters--it's centuries later than the first trilogy--but I think you'll really enjoy it! Wax and Wayne are great characters.