Hm... I like the Ejecta Barrier spell, but it seems a bit overpowered. A constant stream of upward-flying rock & material would require a lot of focus. Something like that would have to be limited to a fairly short period of time, and/or require the caster to keep casting it every turn for it to remain active (i.e. if you cast it, it lasts for takes 1 turn to cast and is active for 3 turns, and you can't use your caster--to move or cast other spells--until the spell duration ends.
Either that, or break the concept up into two different spells. I think for an earth-wall type spell, just a wall of rock would work better. To incorporate some elements of your idea into it, it could be "destroyed" by attacking it (like Ice Wall)... but doing so risks falling rocks that do damage. Which could be dual-cast (as per my earlier idea) w/ water to create a mud-wall... that can be destroyed much easier, but leaves a pool of mud once it's destroyed that's hard to pass through.
Then, for the more raw, offensive aspect... basically just an underground explosion. "Eruption." Rock & dirt gets blasted upward, doing small area-of-effect damage (similar function to fireball, but more damage and smallar AoE). Dual-casting with fire would yield a magma eruption, which does heavy damage to one target, and results in a bunch of ash in the air that diminshes visibility for the rest of the turn and the next turn. W/ wind would decrease damage, increase base AoE. With water would do very little damage w/ no AoE, but create a mud-field.
...God, I love speculating about this.
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Anyway, the rest of my ideas I didn't have time to post before:
(O) Evaporate--remove moisture from an area. Used to stop rain and clear puddles... but can also be used to weaken enemies. Combined with fire, it acts like a "poison", slowly weakening the enemy more and more each turn w/ a random chance of spontaneously combusting that increases with each turn.
(O) Wind Avatar--generates wind around the caster, greatly improving speed and granting additional AP. Combined with fire, it increases attack & damages the caster slightly.
(O) Water Avatar--manipulates the fluid in a living being. Can be cast on ally to slightly increase all stats; can be cast on enemies to slightly decrease all stats. A basic buff/debuff spell. Leaves characters--both enemy and ally--weak to fire-element and strong to ice-element (i.e. increased blood-flow, higher internal body temperature).
(O) Lightning Avatar--generates a massive static charge around the ally or enemy character, which has a random chance of hitting random nearby characters for three turns. Chance of ability activating & range of ability increases with each turn, but damage decreases. Static shock only occurs once.
(O) Atlas' Catapult--a heavy Earth-spell that fires an enormous rock up into the air, which hits the ground the next turn for massive area-of-effect damage. It has an accuracy-modifier, so it may or may not land where it's supposed to--player can dual-cast with wind to increase accuracy; enemies can use wind magic to decrease accuracy. So if you use it against powerful wind-mages, you may find it falling on you instead of the enemy. Could be dual-cast w/ fire for increased AoE; could be dual-cast w/ lighning to generate a single massive thunderbolt on a random enemy within the rock's falling area.
(O) Dragon Fire--special dual-cast spell of wind + fire... breathe out a cone of flame incinerating everything within it. Decreases caster AP in half for rest of combat.
(O) Stone Skin--special dual-cast spell of water + earth... covers the caster in stone and mud. Greatly increases defense, but greatly reduces speed/mobility.
(O) Black Water--special dual-cast spell of water + earth... creates an "eruption" of oil on the battlelfield. Like a puddle, but can be set on fire w/ fire magic (for much more damage than regular fire spell).
(O) Shredder--special dual-cast spell of wind + earth. A bunch of jagged rocks (like shards of obsidian) spin around the caster in a circular motion (like that one D&D spell). They deal damage to anything near the caster, and have a random chance of deflecting attacks (ranged & melee).
(O) Blizzard--special dual-cast spell of wind + water. Generates snowfall that decreases accuracy of all units; creates snow field that greatly reduces speed of all units on top of it.
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Regarding the "special" field ideas--like magma as a super-fire, snow as a super-water... you'd have to use snow to get rid of lava and vice versa (rain wouldn't put out molten rock, after all). Additionally, getting rid of a field wouldn't remove it from the map, just downgrade it. So, for example, if enemy creates a field of snow, the player would have to cast a lava-type special spell to melt it... but once the snow is gone, there's a pool of water instead.
