ElementalFanatics.com would cause me to assume the site is associated with CivFanatics.com, one of the two largest Civilization fansites. Unless Spartan also happens to be Thunderfall, we may want to avoid this.
I agree that anything with planet-, -source, -vault, etc. would be too similar to established gaming networks. What about ElementalShard.com? We know we're getting shards, we know they're elemental, and I don't think there's any "Shard Gaming Network" out there.
I agree on elementalfanatics.com- too close to civfanatics.com
I really like shardsofelemental.com . Elementalshard.com would work, too, but some people might think it's Elementals Hard. We'd have to make sure it was capitalized, so www.ElementalShard.com .
Other ideas I have:
*6elements.com or elements6.com (there are technically 5, but since life/death are on opposite ends, it's like there are really 6)
I haven't checked to see if any of these are already registered, though.