Okay. This one's simple.
Story, Arcade and Rampage Modes. Story mode is a series of missions that can be played as Godzilla, Arcade is your traditional fighter, and Rampage is destruction with a time limit depending on the city.
Monsters would be Godzilla, Anguirus, Rodan, Mothra, King Ghidorah, Destoroyah, MechaGodzilla, Gigan, Megalon, SpaceGodzilla, Hedorah and Biollante. Small roster, but it has all of the fan favorites and the only monsters I could imagine being clones are the Godzillas. Godzilla would be a hybrid of the G90s and G2K designs. Anguirus and Hedorah would have their Showa appearances, Mothra and Gigan are Millennium, everyone else is Heisei. Godzilla 1954 would be available for a single story mission since his moveset would not be unique. Same for Burning Godzilla. Occasionally, a level will require you to play as a different monster, but you'll still be Godzilla roughly 90% of the time. MechaG would be faithful except for eyebeams and finger missiles for the sake of a little fun. All monsters would be on a 60 meter scale.
Cities would only include film locations... Tokyo, Osaka, New York, Sydney, Outskirts, Monster Island, etc. There'd be day and night variations, as well as a separate apocalyptic variation.
Missions would each be based slightly on a film storyline... examples of one for each, but there'd be three or so variations of each, and story mode would randomly select one of the variations. The basic storyline would involve the Global Defense Force's attempts to exterminate Godzilla using MechaGodzilla while other monsters crop up. Keep note though, the missions wouldn't be 100% faithful, so there would some changes for the sake of gaming interest, particularly with films involving monsters not ingame i.e. MechaGodzilla duology would still use Heisei version, Titanosaurus/Jet Jaguar, etc. would be replaced.
Each level would unlock a small reward - a new monster for Arcade Mode, a new arena for either Arcade or Destruction mode, a piece of concept art, a trailer, etc.
Day 1: Up From the Depths
Reward: Unlocks Tokyo for Destruction Mode.
Destroy as much of Tokyo as possible within the time limit (X minutes) while playing as Godzilla 1954, without the military depleting your health. Then, return to the sea in the time allotted again without losing your full health bar.
- Regular: night-time; B&W; full area and regular time; military on way
- Daytime: color; less of the arena and less time total; military already present
Day 2: Counterattack
Base: Godzilla Raids Again
Reward: Unlocks Osaka for Destruction Mode. (Note: Anguirus is unlocked later.)
Defeat Anguirus on Monster Island before fleeing to Osaka; destroy as much of Osaka as possible in the time allotted without your health being depleted by the military; defeat Anguirus again at the Osaka Castle without being killed.
- Regular: night-time; B&W; full area; military
- Oceanic: First battle takes place in an open water level rather than on Monster Island
- Arcade: night-time, color, partial area, no military.
So on and so forth. I'll put up more examples as time goes.