I do have a story in mind, though I haven't built it into the game yet - it will be a campaign made up of missions, each of which you are placed into a unique scenario (not just another, build your base and army up and gather as many resources as possible - well, not JUST that. You may be limited with units, or unit types, the terrain may be particularly hazardous or the weather may be rough (some units will have night vision/IR for improved weather performance). Hopefully you'll consider it a good campaign!

Yes, you will absolutely have access to a sandbox mode (or several) where you can experiment to your hearts desire without having to worry about a thing. Blow stuff up, spawn units, mess with and regenerate sections of the terrain - definitely! I'll probably have sandbox things unlock as you progress through the game, though it'll likely be fully available (at least what I've built in) in the Prototype and Beta for testing purposes.

There won't be any reliance on multiplayer (of any kind) - the game will definitely stand alone if someone wants to play it just themselves in all the different game types (campaign, sandbox, skirmish, and I am envisioning a survival type as well - either where you are just given a number of fighting units, or you are bombarded by wave after wave of enemy).