My idea for a series is Music. The reason behind this is that Lego can expand more to music lovers with instruments that have been built. It shows love for Legos, and music. The Acoustic Guitar took me about 2 days to complete because I was having trouble with the design, and it doesn't have string on because I don't know how to add it in in LDD, but you get the picture.
The Mini Piano took me 2 days to complete as well. I was going for the display look for it because you can't really make noise with it, but it's a nice idea to have as a build.