First off, does one exist?

If not, is there any interest in creating a traditional, story-driven campaign for SoaSE?

Campaign modes exist for two reasons- first, they provide a reason for people to play. Start out fighting weaker battles, keep working against harder and harder enemies until you finally win against the ultimate foe, whoever that might be. Or in some cases, win well enough to ensure your faction can fight again somewhere down the line (See also: Starcraft expansion pack)

Second, they give players a softer introduction to the game. A newbie jumping into ungoverned online play for the first time is going to get shredded. This will probably happen at least the first ten games they play. It's the nature of the beast. A newbie jumping into a campaign mode game will probably win, especially in the first handful of scenarios. Losses may occur, but overall it's much more enjoyable than getting beat down again and again while you figure out which buttons to hit as fast as you can.

Part of that softer introduction is a limited scope of units, research possibilities and area to explore. As time goes by, they get a bigger, rougher sandbox to play in. Drop a newbie into a 10-sided, multi-star, Unfair-difficulty map with pirates and unlocked alliances, and they will usually not do well. Drop a veteran into the same scenario and he'll spend an afternoon cheerfully beating down the AI with a totally different technique from the one he used yesterday.

As far as I know, that gradually sloped, well-plotted introduction to the game doesn't exist. Making it might be fun. Anyone interested?