WieWaldi I strongly vote to count an advanced piece as more pieces with a simple rule: Each movement section/part/chorus/whatever counts as a single piece. This does not address easy vs difficult, but it addresses short vs long. About the difficulty level, this challenge is to master 40 pieces in a year, meaning you must do whatever is easy enough for you to reach 40. Remember - the concept behind this this challenge is based upon the "surprising power of quantity" [see link in original post].
I like that idea, it's a way of tweaking things so that people who are playing more advanced pieces can credit themselves for tackling longer, complex pieces. For instance, a Chopin piece might easily be 3 or 4 times more difficult than a piece from Alfred's book 1, so could justifiably count as 3 or 4 pieces for the purpose of this challenge.
@BartK I see the reasons why you recommend sticking to the original rationale of the concept to gain the maximum benefits, but I can also see why people might like to adapt the concept to suit their own playing level/preferred practice methods - it's rarely the case that "one size fits all".
I think it's fine to adjust the target number down a little, to take personal circumstances/difficulty factors into account. Maybe I should change the thread title to read "40(ish) Pieces" or "30-40 Pieces"?
@Animisha If you prefer to work on stretch pieces I can see why it might be hard to reach the target of 40 (easy) pieces a year in addition to those stretch pieces. Perhaps you could set your own personal target lower - how about 25 a year? Is it doable to play one easy piece over the course of 2 weeks, as well as your other pieces? The easy pieces might be a refreshing break from the stretch pieces.