I've been thinking up a few random ideas. These are just really basic and some are very undeveloped, but I'll share them anyway in the hope that someone gets inspired.
1d Roguelike
Like a normal roguelike, but just played in one seriously long corridor! The idea is that dungeon generation is ignored, so the combat side must be made significantly more interesting to compensate.
Achievement Roguelike
A normal roguelike that's filled with hundreds of steamlike achievemnts, like killing five creatures in five turns, completing the game wthout killing anything, dying within the first 3 turns or collecting £1,000,000 in treasure.
Character Creation Roguelike
The whole game is just creating your character. You win once you've made the character!
Civilian Roguelike
You're a civilian who has accidentally fallen very deeply into a dungeon. There's creatures and adventurers everywhere and you're trying to avoid everything and use stealth to escape. There's no levelling, you just stay as a useless civilian the whole time.
Doctor Roguelike
You get turned into a tiny doctor and then injected into a human. (Yay Futurama!) So the "rooms" would be the organs (heart, pancreas, lungs, etc) and the "corridors" would be blood vessels and stuff. The patient has a different illness each time. You must navigate to the location of the illness, fighting viruses/germs/etc along the way. You can pick up items like healthy blood cells and use them to cure things later.
Dynamic Roguelike
Difficulty adjusts based on how well you're doing.
Environment Roguelike
No creature enemies because the challenge is given by the environments instead. Traps, crumbling cliffs, flooding rivers, rickety bridges, rolling boulder traps, pits...
Exploration Roguelike
The game is easy to win, but the idea is to explore as much as possible to get treasure. Getting treasure would be dangerous though...
Interactive Fiction Roguelike
A roguelike played through text commands and with text descriptions.
Minigame Roguelike
Everything is a minigame. Unlocking doors opens the lock-picking minigame. Disabling a trap opens the disable-trap minigame. etc, etc
Pacifist Roguelike
Can't kill anything directly, but you can kill things indirectly: summoning allies, using levers to set off traps, spreading fires, flooding areas, enclosing enemies with walls...
Powergamer Roguelike
Everything is really over-the-top. Cast fire-volcanoes at level 1. Find the holy grail on the second level of the dungeon. Enemies could be undead celestial ancient zombie dragon gods, but they die instantly from your holy rustproof ultimate-fire-slaying nevermiss greatsword of tamora.
Race Roguelike
The idea is to complete the game as fast as possible. Creatures would slow you down, like spiders that makes webs. Spells would be geared towards speed, like wall destruction and teleportation. Perhaps games could be generated from user-specified random seeds to allow the player to retry games and improve their scores. This would also allow different players to compete against each other by sharing their seed. Secret doors would be good. I think the whole map should be visible all the time to allow the player to choose the best route. Perhaps the amount of mana given to the player after each level is based on how fast they complete the level, to work as an incentive to completing levels quickly.
Reverse Roguelike
You place the creatures and the computer plays the adventurers. Like Dungeon Keeper.
Screensaver Roguelike
Everything is automated: adventurers, creatures, etc.
Sideview Roguelike
Sideview roguelike with ladders, gravity, falling on enemies to kill them, etc. It could be set in a tower. This could be one continous game without levels, if extra floors are generated as the player gets closer to them and distant floors are deleted as necessary.
Sniper Roguelike
You can kill creatures from far away, but you're really vulnerable when they are close. Clever use of terrain and stealth is needed to survive.
Sponsored Roguelike
An "adveroguelike" where everything has been sponsored in an over-the-top way. e.g. Pepsi cans instead of potions. Harry Potter branded spellbooks. Environments are McDonalds restaurants instead of dungeons. All the enemies are sponsored by the dentists association: plaque, gum disease, tooth decay...!
Story Roguelike
Every action you do (except movement) creates a sentence describing it. At the end of each game you are presented with a story of your adventure. For example: "Claude entered the temple of the moon, when suddenly a giant bat flew down from the ceiling! He thrust forth his spear into one of it's wings. Unable to fly the bat dropped to the ground helplessly. The giant bat let out a sonic noise to alert other bats as Claude plunged a sword into it's brain."
Unfair Roguelike
It's permanently Friday the 13th: Instant-death traps, dragons on level 1, cursed items, random poisoned potions, unexpected falling boulders, death by random spell failure... the aim of the game is to discover all the ways to die.
Willow the Vampire Slayer Roguelike
You play Willow who fights vampires+demons using magic in graveyards+crypts+masoleums. You're friends will come along to help: Giles will provide useful information. Buffy and Faith can fight. Xander can use army weapons. Angel and Spike have their vampire abilities. Dawn can annoy people to death. Anyanka can grant wishes. Oz can turn into a werewolf. Cordelia can throw shoes at enemies. Tara can help Willow to make seriously powerful spells. It'll have lots of big bads from the show, like The Master, Glory and The Mayor.
