the theme should be New Year's... if there are several rounds, then you can have different genres (fantasy, science fiction, or nature, humanoids, etc., and one New Year's theme)...
and of course, the drawings should be original creations by the artist... (because sometimes you look at the Art Gallery and see a drawn copy of some artwork)...
P.S. I often drew units from Heroes (now less often), but they weren't similar to the original because I take the idea, not the picture with all the details.