Usually in Jams themes are revealed on the day the jam starts, so you also have to come up with the concept on the fly.
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Yeah but the idea for this Jam didn't seem to be that one. It was gonna be one of the 4 themes that are on the main page, you're just trying to do something completely different. The idea was to have the four generic themes up for voting and one would be chosen and we would have to make games using that generic theme. Not a consistent theme like you said. This is not Ludum Dare. If you want consistent themes, suggest the idea WEEKS PRIOR to the jam. Not one day.
I mean, isn't that kinda silly? Oh, so the theme will be Wild west, you have one month to make mockups and think about ideas. Then one day before the jam, you say "oh so the wild west theme will be "Aliens and Cowboys. Must have aliens in the game" " Well, there goes my idea for a game without aliens that I've been writing for a whole month just because someone just decided that the theme should be less general. That just makes more people upset and not want to participate in the jam because it cuts creativity, you know? The palette is already a challenge, why do you want to complicate people's lives even more?
Theme's for jams are generally to BOOST creativity, not to cut it down. So for a theme to be announced on short notice, it will make you design a game from scratch including concept. This means you have a lot of free reign in your design, instead of baking/nailing down every detail WEEKS before (which DOES cut down creativity).
In fact, you could even start developing the game then already, that's why the theme is always announced a little before the kick-off, so everybody has exactly the same amount of time and work in their projects.
All in all, I think you can still make one of the games you already designed, since theme isn't the only thing games are rated on. In fact, it's only 1 of 6-7 things the game is rated on. If you still make a good game with the limitations presented, theme shouldn't be a problem.
And also, 2 weeks is more than enough to write/design/develop a proper game of the expected quality.
That's how I feel anyway.