For the first three competitions we chose a classic arcade game and then pulled out select tropes to set the theme, but recently we've been just coming up with our own themes, so I'm assuming everyone still wants to do it this way. If not, please feel free to speak up.
In Competition #5, we made an entire list of themes we came up with, so I'm putting it here (minus the theme we chose that time):
1. Circles, Arcs, and Curves. In one form or fashion, your game includes circles, arcs, curves, ellipses, spheres, etc. This may be a circular playing area, things that travel in circles, circular playing pieces, or something else. Just something that isn't a straight line as an important piece of the game.
2. Choices. Some aspect of the game involves choice, whether it be in playstyle, in story progression, or purely visual appearance.
3. The Button. Perhaps your game is a one-button game. (Making a game where you've only got one button is an interesting thought to begin with.) Maybe the game controls a ship, and all you can do is choose when to fire. Maybe you have lots of buttons, but one that does something spectacular. Maybe it's a trivia game or a reflex game where it takes skill to push the button at just the right time to beat your competition. Maybe it's a thematic/storytelling element with a button that detonates a nuclear weapon. Whatever it is, somehow a button is key.
4. Overkill. Have something happen that is overkill for what it intends to accomplish. Like using a nuclear bomb on an ant, putting a gallon of hot sauce on salad, slicing a man into hundreds of pieces when just one or two slices would have been plenty.
5. Sell it! Basically this would be a double meaning, first being that to win.. you actually have to have a way for someone to physically buy it, I think there are some stuff on paypal for setting up things like that, so even if its just posting a "Buy" paypal link, that allows us to pay $1 and get the download link, you have to make sure it is sellable and not under any licenses..
The other meaning is that you have to make a game where someone sells something that is part of the game.. so you could do a lemonade stand type thing, or you could do a hero game where the first thing you do is buy your sword, and the last thing you do is sell it.
6. Alternate. This word has two meanings. First, to switch back and forth between two (or more) things. Second, a substitute or backup. This could be like switching between characters in Donkey Kong Country, blue and orange portals in Portal, Player 1 and Player 2 taking turns like in many classic arcade games, alternative paths through the game like Castlevania, or alternate endings. This is admittedly somewhat closely related to the Choices theme, but I think there are enough differences that it will inspire a different set of game ideas.
7. Change. Something (or things) in your game is in constant flux that affects how you play or needs to be taken into account when playing. In a stealth game, this could be enemy guards going through periods of extra attentiveness or incredible inattentiveness that may need to be waited for before moving, or in an action game, the final boss' shield constantly changes in its strength, which will either block or let attacks through.
8: Stars. Maybe a space themed game, maybe it's movie stars, maybe you collect or earn stars, or maybe it's a thematic element in an RPG of some sort. Stars seem to simultaneously represent something unattainable, yet also seem to represent achievement.
9: Asymmetry. A game that is asymmetric or unbalanced in some way or another. Perhaps different teams or people use different tools to play the same game. (Like the 3 vs. 1 mini-games in Mario Party.) Perhaps its an imbalance in the forces of good vs. evil, and your job is to "balance the force," to coin a phrase. At any rate, one of these things is not like the other.
Last time we had an official vote to choose, but we still have a while before the competition will probably start and usually we can just talk to come to a decision, so let's give this a couple weeks for people to throw in their opinion.