I always like to look at previous themes. I don't think there's anything wrong with dupe or similar themes as previous but I prefer to suggest something new.

Previous Themes:

• Indirect interaction
• Guardian
• Construction/destruction (sheep)
• Preparation – Set it up, let it go
• Infection
• Random
• Light and darkness
• Growth
• Swarms
• Moon/anti-text
• Build the level you play
• Chain reaction
• Weird/unexpected/surprise
• Minimalist
• The tower (owls)
• Roads
• Advancing wall of doom
• Caverns
• Exploration
• Islands
• Enemies as weapons
• Discovery
• It's dangerous to go alone! Take this!
• Escape
• Alone (kitten challenge)
• Tiny world
• Evolution
• You are the villain (goat)
• Minimalism (potato)
• 10 seconds
• You only get one
• Beneath the surface
• Connected Worlds
• Entire Game on One Screen
• An Unconventional Weapon
• You are the Monster
• Growing/two button controls
• Shapeshift
• Ancient Technology
• One Room
• A Small World
• Running out of Power
• The more you have, the worse it is
• Combine two incompatible genres
• Running out of space
• Sacrifices must be made
• Your life is currency
• Start with nothing
• Keep it alive
• Stuck in a loop
• Deeper and Deeper
• Unstable
• Delay the inevitable
• Every 10 seconds
• Harvest