Yes, and you do that by making a fun game. Strategy gamers don't care about story, they want to make their own. Compare the success of the civ series (the only "story" being a vague tie-in to history) to... um... I can't really think of "lore" heavy grand strategy games. I guess the scope of a GRAND strategy game is beyond a "story". Look at MoM, the game this is the spiritual successor to. There was NO story, and you could customize fairly extensively (it wasn't spore, but for its time it was pretty open).
Story? Yes, story goes with campaign, which I think is secondary. I'm talking about fleshing out the factions themselves. You know, the ones people will be playing against all the time even if not as in sandbox mode?
And Civ IV kind of proves my point. The devs didn't need to work very hard to make the factions feel real, believable and vivid because they are Mongolia, the Aztecs, Portugal, Russia, Germany and so on. The very same civilizations that everyone knows either from history, current affairs or both, and we all already have impressions of them in our heads. Not only that, but unless you select random AI personalities they behave roughly like you might expect them to (or at least, how you might expect their leaders to behave). Gandhi is polite and hard to anger, Genghis Khan isn't happy unless he's at war, etc. There is something appealing about playing out an alternate version of history.
IMO this adds quite a bit. Sure, once you've played through often enough it's nice to mix things up, turn on random AI personalities for a surprise, and so on. But I think it makes it much easier to get drawn into a game and really feel like the world you're playing in isn't just some empty shell if you can relate to its inhabitants. This is extraordinarily easy for games like Civ IV for reasons stated above, but actually requires some work in games like Elemental.
Even more importantly, though, is that the factions all feel finished and diverse; so that playing as any one of them results in a very different game. If they aren't different in interesting and significant ways, the game just feels bland. There is little point of selecting one of them as your faction if they're all pretty much the same, but what's even worse is that it means all your opponents in-game will all be more or less the same. The original GC II solved this by giving each race a very different personality, and in expansions gave them different tech trees which made all the difference in the world. Civ IV actually suffers because of this: even where races do differ, they differ in relatively bland ways. Yay 15% more industry... Meh
Civ IV gets away with it I think partly because it does so many other aspects of the genre so well, and there isn't really anything else to compare it to! What other modern 4X strategy games can you think of besides Civ IV and GC II? Not to mention that my generation of strategy gamers grew up on Civ titles, and therefore like me most will buy any sequel regardless of what anyone tells them. Just like I bought HoMM V, even though people I trust told me it wasn't worth it (and they were right); but I've been playing HoMM since II, and wasn't going to let anything like two bad sequels in a row get in my way. Elemental does not have that going for it.
Modding and customization keeps a game alive in the long-term, but if the core content isn't good enough to keep people interested then there won't be many people sticking around to keep it going. Not to mention it isn't like Elemental will come with no modding possibilities. Hell there's a whole beta phase devoted to modding! We're just talking about whether Stardock focuses its assets on the stock factions, or sacrifices some of those assets to give us more in-game faction/race customization. Something which they can do later in an expansion, anyway...