Get a pen and paper.
Your game will be made up of three elements; your character, the environment, and the gameplay.
You currently have a vague idea of what you want your environment to be; WW3. Now think about where and when you want to set it, and what implications this will have on your game. Setting it in the present (i.e. tomorrow) is good because you have access to lots of contemporary assets on the forums, in the store, and in the DLC model packs. But you may want to think about where you set your game a little more thoroughly so you can give the game a unified look and feel. Is it set in America during WW3? Is it set in China during the aftermath of WW3? Is it set in Africa during events that lead up to WW3?
Next, spend a while thinking about character. Who do you play as? What's their story? And, more importantly, what is their journey in the game? Are they a new recruit at the beginning of the game who progresses to a captain by the end? Give your character a goal; this can either be a physical goal like 'reach this bridge by nightfall' or an internal goal 'get a promotion to captain', 'uncover the mystery' etc. Then place obstacles between your character and their goal. Conflict is drama, and drama is what engages an audience.
Now that you have these two things, they can inform your gameplay; what do you actually do in your game? If you're a soldier in post-WW3 America, there's probably going to be a lot of shooting involved in Malls, airports, streets, industrial complexes...probably not much magic in forests! But if your character is a doctor in pre-war Ghana, the gameplay might involve healing people rather than shooting them, and take place in wild open maps with lots of trees and natural elements. This is where Game Guru's own internal limitations might start to get in the way but most challenges can be overcome with scripting and a lot of help from the folks on these forums.
Once you have your character, environment and gameplay, write a storyline. Work out what obstacles the player must face each level and how they overcome them, where the level is set, and when. Work out how this might look as a series of levels. Level 1 could be pre-war, level 2 could be during the war, level 3 could be afterwards. Realise that games work by constantly challenging the player. Introduce new gameplay ideas throughout the game but make sure you develop ones you've already established; players get a kick out of knowing how to do something. For example, in level 1 you teach the player to jump, in level 2 they have to jump up to something to progress to level 3. The hardest part here is tempering your imagination and ambition. Few indie developers ever finish a game they plan to have 30 levels.
And there you have it; a written outline of your game. All that remains now is for you to actually start building it!
AE