So, just winging it here - a lot of this is going off of the Steampunk idea:
Light and Shadow
Contest focusing on lighting, or just light or shadows. Not a good vs. evil idea.
Either something based on historical events, settings, or subjects that are antiquated or aged in modern day.
Focus on the sky, things in the sky, or things revolving around it.
Sci-fi/advanced tech in the past or sci-fi based off of older sci-fi/futuristic concepts. Could make the Steampunk contest broader if it turns out too specific, or do a different kind of retrofuturism (e.g. Diesel punk, Zeerust, certain kinds of cyberpunk). This could also be an alternate reality subject.
Contest that could mess with the idea of identity, how much it matters (if at all), how it affects perspective, etc. etc (someone who has inaccurate or absent memories, or sees themselves as something they aren't, etc.)
Use Another Artist's Style
Not much to it, just have the entry's use a different style or make an image based off of that style. Could be how the artist paints, draws, etc., what materials they use, what the artist usually draws, the tone, or whatever else.