I'm currently working on Gunfist Gaiden. A top down shooter in the old style of Night Stalker, Dark Cavern, Berzerk, Wizard of Wor etc...Think Intellivision/Atari 2600 graphics running around corridors shooting robots with randomized behavior and stats.
Hoping to add a new robot each level and explaining the robot's 'mods' between levels. Real time action. Multiplayer Co-Op. Upgrade between levels like in Fuel (Ido), R.C. Pro-Am (NES) and Off Road (NES). Pick up crystal to buy upgrades, I swear Fuel wasn't the first game to do this, so technically I'm not ripping Ido off. Technically.
Stretch goals include various fist upgrades (scatter shot, pierce, explode, bounce, etc), armor upgrades (1 shot bounces off, 2 shots, 3), speed, etc...Maybe add special equipment 'packs' like stealth pack, nitro speed, shield pack and what not. Packs would work on a cool down.
Another stretch goal is to have a limited number of bombs. Purchase them between levels on the upgrade screen. You shoot them, they stick to walls and explode when a bot gets close. They destroy terrain.
Had a wild idea that some levels would include gravity, making them platform like. Pressing up would use your jets. So no need to make sure a player can jump from platform to platform, since you can fly as high as you like. Imagine a Rogue or Nethack level with gravity pulling the player straight down. No real need to alter the level generation that much, AI is the real challenge.
Super stretch goals would include a player vs player battle arena mode. Giant boss robots. And a 'build your own' mode where you can draw a robot (8x8 pixels, 8 colors), assign mods/traits, set it's difficulty (what level it should appear). Also design your own levels with drag and drop. Practice mode where you can set the difficulty, terrain tile set, enemy theme and architecture. Maybe even set the robot type.
You can see I've thought a lot about this game. Right now I have a guy running around, a robot that doesn't move but blows up nicely when you shoot him. :-)
