Well, unless you make them implement differently, I don't think it is a good use of the space (limit of spell spheres and attempting to keep it to 1 spell per level) to flood the market with attack spells.
The options would be to make it FAR more common to have a resistance and a weakness to damage types. Then even if all the spells work the same, they are not completely interchangeable, you need the right one for the right situation.
Ideally, if not done by making resistances/weaknesses very common (and returning the the rock/paper/scissors aspect, but now with magic), make it so they are not all just "Races" of attack spells. Like water being +50% across rivers or from the coast, sun being +50% in desert.... It would be best to make it so each one was mechanically different, then you run into problems for what you can code.
Like lightning. It can be as proposed above, stronger against weapon promotions. But can you base something off the opponent's promotions? It could also be that it has a larger range than fire, but the strength reduces with range. But can that be done? Or just give it Blitz to simulate chain lightning, or huge collateral damage.
Ideally, Earth would be able to reduce city defense, Lightning would do collateral damage, and fire would do damage over time. But what about all the less obvious spheres? The more spheres that get some kind of attack ability, the more the other spheres desire them, or some other compensation in place of it...
I think that it is fine as is. Yes, if you are looking in terms of fighting, then Fire is the standout as attack, but many people also enjoy death and the upper end of some other spheres... so I think it is nicely set up in that respect.
But Civ isn't supposed to always boil down to a fight. The ideal game you wind up never really having war. Or only short wars to achieve political agenda. And as such, it is remarkable how many options have been worked into the magic system that are NOT fighting oriented. It is easy for us to think of ways to hit stuff, but hard to think of worthwhile ways to go about normal life. So I am quite fond of the work done with the magic system so far, and how difficult it can be to make your decisions for anything but fighting.
But yes, it all boils down to: this is a game, and once your civ is wiped out, you just start up a new game and go at it again. Hence there is no natural aversion to war and fighting. When you have 2 cities left you PREFER to be wiped out rather than sit around scraping out a meager existence. Unless you are lucky enough to be playing a Diplogame, in which case you aren't doing so hot if someone knocked you down to 2 cities
