I don't set up challenges of my own, but it is interesting to read everyone else's challenges (and their progress.)
This year, I did accomplish a couple of things that I have been working on for a while. One, is that I have read nearly all of the books that I purchased from the now-closed Discover a New Love store. (I have two books, a complete series, to finish.) I can not say the same thing for my purchases from Fictionwise, Books on Board, or Sony.
Two, I have finished all of the books that I purchased from Smashwords; although they are not a closed store, I have not been purchasing books at all except to complete series that I've started or because they appeared discounted in my BookBub list, where Smashwords is not represented. I have a couple of series that I follow and purchase from Smashwords, but I buy them when I'm ready to read them and finish them right away. I wish I could do that with all my stores!
While not really a challenge, next year I'm going to come up with a better solution for my Kobo and B&N books. I'm running out of books that I can download and read in my web browser! (I'm at a point where I've almost given up on purchasing books; there's no point to buying them if I can't download them to read.
At the moment I'm limited to reading them in their respective apps, and I do most of my reading in my web browser -- reading through lunch -- and both have abandoned their web readers.)