Make a campagn where you start off in the Tutorial map which has been destroyed. Dont worry, this hasn't been done befoh wait
My recommendations:
If you know how to edit text files/code, try to integrate your add-ons into the gamemode itself instead of requiring players to download them, I've done this with my deathmatch gamemode. That way players dont have to download several files to play, only a single big one. This also lets you do edits to the add-on files without requiring players to overwite their actual add-ons with your changes.
Try not to overload the players with story, especially if its just expressed through text and they have to sit and read everything. People want to play, make a cool map! Though if you think you can execute a deep story well, go ahead and risk it.
I cant really think of settings too well, but I think one where you stay in somewhat the same place would not only make it easier to build, it would require less bricks than a huge expansive map would, making the game run a lot smoother. I'm reminded of Half-Life 2: Lost Coast. It's short and all takes place in just one map. The default Ninja Challenge also comes to mind.
