A lot of your works that are stand alone novels or seemingly completed stories, you have announced or started working on sequels for. Are there any stories that you feel complete and don't need to work on the same world or characters again? Or do feel there is always some new tale to tell about every world you make?
It's hard, because the way I plot I always have to know what happened before the book and what will happen after the book. Knowing that doesn't mean that I have to continue. It's also hard, though, to say no to fans who are so passionate about a specific project.
The Vin/Elend story is most certainly done. As is the Raoden/Sarene story, as is Siri's story from Warbreaker. So there are completed threads. There might be other stories to tell in those worlds, though, so I'll avoid closing the door on them for now. (That said, it did feel very good to finish The Wheel of Time for good, and look forward to putting some of my own works to rest in a similar way.)
You've mentioned that The Stormlight Archive is broken down into two sets of five books. Is the story arc of the second set of books going to be completely different than the story in the first 5 books?
It will focus mostly on different characters, with some appearances by characters from the first five. I wouldn't call it a different story, more a sequel.
Has Midius found his way to Taldain at some point? If so, will we be able to see him there?
Taldain has seen visits from non-natives.
Does "Alethi" come from or have anything to do with the Greek word for truth or is that just a coincidence?
Alethi is a coincidence. However, it is the sort of coincidence that happens a lot for me in languages, as I often look for a "feel" for a language. Alethi, for example, is a Semitic language mashup with some Mediterranean influence. So I'm not surprised if it means something in the right languages. (I did this with Straff and Elend from Mistborn, looking for Germanic-sounding words and accidentally using two words from German.)
Does Hoid have all of his fingers
Hoid has had fingers chopped off on occasion. I doubt he's kept them around after the new ones grow in.