I'm just gonna gush about "Decay" for a bit.

"Decay" is a wonderful theme.

It does have some more "generic" or "boring" interpretations - "help, our kingdom is decaying" and the like, but that's not all it is! The best themes have creative interpretations mechanically, visually, or thematically, and "decay" has a bunch of options.

Mechanically, anything on a timer is decaying. You can style that in a bunch of creative ways, for example: - plants that die if they spend too long in darkness - cars/machines that rust over time - which you can accelerate with something like rain - a match 3 game where the pieces crumble into dust over time - power that "decays" - you start of strong, but the more you use your power, the faster it runs out - batteries or radiation

There are loads of possibilities and I'm sure you're all much more creative than me.

Then, there are the story-based implications. Anything to do with plants, or something like the Flood in Halo, works nicely, but there's also things like houses falling into disrepair, radiation, or you could go for a general sense of "rot" in something like politics. There are a ton of options.

And finally, there are some INCREDIBLE visual styles that you can pull off with this theme. I mean look at this, from a quick deviantart search: Source

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Obviously stuff of this quality isn't possible in a jam, but you can get a lot of mileage from very little here.