[HoneyBadger]

Water has great underwater summons, including the sea serpents, which are lovely monsters who eat independents for dinner and don't cost much, in levels or gems. Summon up a dozen in the early stages of the game and you won't be sorry. Earth has built-in usefulness because it's got lots of nice early game forged items that you can make en-masse, and earth drakes, which are really tough for low level summons, esp in early era where armor is at a premium. I'm not too impressed by fire magic. It could use some later game boosts, and more variety. It's nice that you can sell the gems for 15 gold a pop, but earth gems are much more valuable, and they're only worth 10. Is there any good reason why the other gems can't be converted to gold? I know it's probably for balance, but pearls, sapphires, emeralds, amethysts, they're all really valuable, especially the pearls and amethysts in ancient times. Still, gold in the game isn't so valuable a resource that the other gems shouldn't be unconvertable.
But I digress, (I do that a lot)
I'd like if you could use mastery in fire to combine more with other paths: more magma, steam, some kind of eldritch death-fire Anthrax spells, maybe something with Astral to make Aether magic? Aether (along with fire, earth, water, air) being one of the Platonic elements. Water, overall, seems to be a little bit more useful, especially considering the ability to freeze your enemies solid. You can set your enemies on fire, but so far I haven't seen this have too much of an effect. Can one troop on fire set nearby troops ablaze? probably, but I haven't seen it yet.
Death to me is the most useful, throughout the game, regardless of nation, with astral and earth tied for second.