Engine and graphics style:
Retro characters and objects, rendered in a doom style way (floating 2d sprites). No walls since that's a tad more tricky to implement code-wise. The player will probably be constricted to a 2D-plane, unable to aim vertically, however there will be jumping and similar events in the vertical axis. Enemies might spawn by landing violently on the game field via some sort of orbital insertion.
-Gameplay-
The player (and enemies) moves around by accelerating to high speeds either by an ice-skating-esque movement scheme, or by using boosters spread out over the map. Space can be used to jump, which is essentially used to quickly nudge the direction of movement, allowing the player to dodge projectiles, etc. Spread out on the map are weapon caches where other weapons can be found. Allthough later weapons can be considered more powerful, there will be tradeoffs to them. The objective of the game is to go to a specific place on the map and do something there, perhaps destroy a spaceship that's preparing for liftoff.
Boosters are also spread out accross the map, these help the player gain speed quickly without having to accelerate via the skating mechanic. They also grant the player a temporary top speed boost. The idea is that the player should plan a route to the goal via the boosters that passes through weapon caches in the very start of the game.
Since I had this idea of letting the players explore the game themselves, without the use of tutorials, there needs to be a clear goal of the game. Thinking about this for a bit lead me into designing a simple yet very efficient map idea, which looks like this:
Blue lines are paths that players are likely to choose.
The bullseye shape will guide players towards the center, where the boss is found. The colors of the bullseye will affect the ground seen by the player ingame, however, I'm note sure I'll be able to make a nice transition, maybe the entire ground will just fade to another color.
-Graphic elements-
An enemy will probably have a total of 10-13 frames, depending on the number of frames for the walking animation. There will essentially only be two animations per character, a walking one and a shooting one. The enemy will be able to be seen from 4 directions, straight in front of, 45 (yup, not 90) degrees from the left, 45 degrees from the right (mirrored 45 degrees from the left), and from behind. This is because most of the time, the enemy will be looking straight towards the player, or trying to turn towards him (45-degree frames here), or sometimes making a u-turn after the player has passed it in high speeds (from behind frames here). The shooting frame will be from the front, since it wouldn't really make sense for any other directions. Any more frames would really be overkill.
In order to enhance the sense of speed, there should be small terrain features like rocks, holes, small vegetation and whatnot spread out across the ground. These are just single frame objects, and it would be nice to vary them by having a bunch to choose from. Larger terrain features like pillars and whatnot could also be placed randomly on the map. These could have collision enabled and force the player to dodge 'em, but I haven't really given them all that much thought yet.
Oh. And guns.
-GUNS?!-
I'm considering a pretty standard loadout, since what makes weapons interesting in FPS games is the weapon feedback imo. The loadout might consist of the following:
Pistol: Starting weapon. Accurate at long range (even though that hardly makes sense, oh well), Pretty high damage, takes maybe around 1-3 shots to kill one enemy. Single fire, you can't hold the mouse button down. About 8 rounds a clip should be allright. Firing extremely quickly should throw your aim off a bit, but not too much.
Shotgun: Excellent in close range encounters against groups of enemies. Piercing possibly? Damage is lowered based on the distance to your target, deals no damage at all when distance is higher than a certain number.
Sub machine gun: Your weapon of choice against multiple enemies at medium range. Not that accurate nor high damage, but the high rate of fire can weaken groups of enemies from a distance. Against single enemies, the pistol is probably a better choice.
Homing missile launcher of doom???: Secret weapon? Would be pretty cool to add an overpowered weapon somewhere, maybe in an untagged weapon cache. High rate of fire (compared to an actual rocket launcher, of course), homing perhaps? Splash damage. Not that long range though.
Enemy weapon: Probably some sort of plasma rifle with semi-slow projectiles that can be dodged. Could be tricky to tweak since all characters will be moving at pretty high speeds. Maybe this could be adressed by having the projectile fire at a speed relative to the firing enemy?
-Damage model-
Since there's no real Y-axis in the game, headshots are impossible. However, a weapon could damage more if it's aimed directly at the center of an enemy:
That guy on to the right... He's dead.