1st - Harry Potter

After reading the tasks I immediately thought of many other things you could do, like the Yule Ball, exploring the castle, eating magical candy, meeting the magical creatures like hippogriffs, thestrals etc... There's just so much you could do! I think this theme could at the very best give the fans of the books the motivation needed to finally improve and get creative with their lucid skills

2nd - Tech Breakthroughs of the 21st century

Really modern and fun tasks! I personally haven't had a lot of modern devices and such in my dreams so this would be something new.

3rd - American Cities

This theme would be good because you don't need background information (right?) to try the tasks. So even if one is not familiar with the other themes, they could at least try this. All the tasks seem fun.