All LD25 themes, with ideas

Swarm - Direct a swarm of bees with the mouse. Sting predators and people enough to protect the hive, but not so much that all the bees are dead.
Shadows - Move things around to make sure something is always in shadow, so nobody sees it.
Parallel Worlds - Platformer where every time you die, play switches to a 'parallel world', where things are very slightly different. Gradually things get very strange.
Reincarnation - Hmmm, kind-of covered this with the above. Maybe instead, each time you come back as a different character, with different abilities. Birds can fly. Trees just slowly grow towards the goal.
Surveillance - Tower-defence-like where you set-up security for a museum exhibit, then have no control while thieves try to break in.
Trapped - You are stuck in a bathroom (or some other room, maybe). You must survive until somebody realises and rescues you. Find things to eat, drink, keep you from going insane from boredom. Plays-out in real time?
Collapsing - Platformer where each time you reach a goal, part of the folds up, giving you a new challenge.
Infection - RTS where you play as the immune system - you cannot see viruses, only the dead cells from when they errupt. You must control the spread of infection for long enough to develop antibodies, while causing as little damage as possible.
Disaster - Play as a rescue worker after an earthquake. You can only hear any survivors when they are very close, each will only live for so long, and you do not know where they are or how many there are. Game ends when you choose to stop searching.
End of the World - There is a huge asteroid heading for Earth, and you must do your best to prevent it colliding and wiping out all life. You cannot win, and the game will not let you play for a second time.
Outer Space - You listen out for radio messages from space. Sometimes you receive one, but it is from so far away that it would be impossible to communicate back.
Construction - Reverse Tetris. Attempt to build the tallest skyscraper out of tetrominoes, without ever creating a line of 5 squares, or they will disappear and the tower will collapse.
Powerless - Adventure-style game where there is a problem presented to you, however despite lots of seemingly-possible solutions, there is actually no way to win and you lose.
Chaos - Game about chaos theory. You are trying to collect small pieces of paper that you have dropped, but every time you move they scatter. I don't like this idea, but I can't think of anything else for 'Chaos'.
Colonize - Play as mould in a house. People will spray you if you are too obvious, so grow in the shadows to build your strength.
Absorb - RPG where every time another character speaks about you, their words affect your stats, like buffs/debuffs.
Inside - Visual novel where you are falsely imprisoned. Avoid attracting negative attention. You can attempt to escape, if you wish.
Stranded - You are like ET, accidentally left alone on an unfamiliar world. Survive until your family return for you. Who do you trust? Gradually learn the language and culture.
Afterlife - RPG where after death, you return as a ghost and can attempt to guide your successor.
Destruction - Sneak into buildings in order to set them on fire. After you are eventually caught, you are taken to court and it is revealed to you how many people died because of your actions.
Invasion - You are a mind-controlling alien. Your aim is to brainwash all the humans in each level, without anybody getting suspicious and alerting the authorities.
Old vs New - Work as an art expert, and identify subtle differences between originals and fakes. Criminals will bribe you if you can convince your boss that a fake is real.
Inverse World - Go through a life backwards, making sure that events line-up with what has already happened.
Mirrors - Identify vampires by attempting to look at them in a mirror. The vampires are wise to this, and will make efforts not to go near reflective objects.
Quarantine - Play a government during a deadly epidemic. Each day, you get a map of how the disease has spread, and you choose where the quarantine will go. It takes time to set-up the quarantine, so it is easy for the disease to escape. Anyone in the quarantine area will be gassed. Save as many people as possible.
Night and Day - Play as a superhero. Stop as much crime as you can, while also not losing your job.
The End is the Beginning - Game starts at a funeral. Guide the protagonist through the stages of grief.
Migration - Flight sim where you are a bird. Carefully manage your energy, keeping in position behind another bird to slipstream.
Deception - You have several different fake identities. Avoid mixing these up and telling lies from other identities.
Guardian - An entire game composed of escort missions, because everyone loves those. After each, the person you were escorting is killed stupidly in a cutscene. See if anyone actually finishes the game.
Unstoppable - Your car can no longer brake. Drive through a busy city killing as few people as possible.
Science - Working as a school chemistry teacher, manufacture and sell drugs and other illegal substances without anyone finding out. If someone starts to suspect you, you must poison them.
Claustrophobia - Turn-based puzzle game, where you navigate through a crowd without ever becoming surrounded and unable to move.
Ghosts - You are a ghost in a big house. People keep moving in, and you must convince them they are insane by moving things around when they are not looking. If they see you enough, they call in the ghostbusters and you lose.
Time Loop - Navigate through levels by entering x commands, which are then repeated over and over.
Mind Control - Transfer your conciousness to other people in order to open doors and perform tasks. However, while controlling someone else, you must press certain keys to make sure that your real body continues to breathe.
Fortress - Manage a besieged fortress for as long as possible, with the problem that you must occasionally open your gates for pizza delivery men.
Transformation - Defeat a series of obstacles, each time by choosing an item to be transformed into a cat.
Dystopia - Keep your citizens in fear of you enough that they dare not rebel. This is done by bluffing in official speeches. If you are caught, fear drops and you risk a rebellion.
Alter Ego - You are a werewolf. Each day, carefully arrange things so that you don't harm your family in werewolf form during the night. (eg, leave a steak out, but with sleeping pills in, so that you eat that.)
Symbiosis - You can choose to give a symbiotic organism various amounts of your food. The bigger it gets, the more it benefits you, but if it gets much larger than you it may turn on you.
You Are The Villain - This was the theme, can't be bothered.
Survival - Most of these ideas are already about survival... Maybe, you are an ambulance driver, and must get to people as quickly as possible but without crashing. Sometimes you do not have enough time before a person dies, and it is impossible to be fast enough to save them.
Growth - Run a plant nursery, genetically modifying plants for desirable traits, and potentially unleashing a plague of deadly mutant vegetables.
Ocean - You are the ocean. Manipulate your waves to guide little boats to the shore.
Journey - Navigate through an unfamiliar area with only a misleading sat-nav.
Underworld - Wander through sewers, uncovering personal items that people had lost. And sewage.
Salvage - Search through a rubbish tip for anything with value, that you can sell to buy food for your family. Time limited, and you can only carry so much at a time.