Hi there, thank you for reminding! I've searched for several licenses and I regard CC-BY4 best suit my wish for this asset -- I hope people can use my work whatever they want as long as they credit me haha. But for other projects after this, I might change the license to suit my goal.
For pixel asset ideas: Maybe "antagonists"? Many pixel RPGs have monsters/enemies so drawing some antagonists might be great. This theme is also wide enough as it doesn't limit what exactly you have to draw (it can be monsters, people, or even evil environments/items) so people can have more freedom when drawing.