New magic / spell mechanics
Channeling spells: Useable for very powerful spells with a duration. (Example: the Earth elemental I outlined below)
These spells cost mana "upkeep" every turn and only last as long as the caster keeps casting them. During that time he can do nothing but walk slowly. That locks the caster into doing nothing else so it better be worth it.
Tactical casting time:
You can easily cast that Fireball in your turn but for big juju like Chain Lightning you would require more than 1 turn.
So when the opponent starts chanting (some light effect sparklies to give you a general idea of which element), you know that something nasty is going to happen. And you have a chance to react to it.
A fast casting stun to interrupt the cast, lots and lots of arrows... there could be magical duels instead of instant annihilation before ever getting to move.
There would be an interrupt prevention / channeling / concentration skill or spell to keep casting through such interruption attempts.
The lvl 1 caster would easily be interrupted so would probably stick to smaller scale spells but the mighty wizard might still open with Chain Lightning, gambling on just toughing it out.
This would not only benefit "caster duels". Troops would have a chance to spread out when the enemy starts with the big area juju.
Right now the cards are all stacked for the magic users. See a troop concentration? Cast instant KABOOM.
In fact, you could have more "fun" with this if AE spells had to be targeted at casting time. When the spell finishes, the targets may no longer be there.
This mechanic could also be used for slow loading weapons like catapults or crossbows, only letting them fire ever 2nd round.
Crossbows would be a good weapon to pierce metal armor (high attack) but obviously slow to fire.
This would extend ranged weapons into the late game (crossbows, heavy crossbows) and not stop the ranged weapon research tree dead after the first entry.
More Area of Effect shapes: Square (1x1, 2x2, 3x3, etc), Radius (3x3, clip corners to make round effects), Cone (generally triangular, expands wide in two directions), Line (2x4, 1x6, etc). This would greatly increase the versatility for different spells to be put into the game, Fireball (round) Cone of Fire (5 square cone), Burning Hand (2 square cone) Polar Ray (1x6 line)...you get the idea.
Point Blank AE (PBAE) that cast an AE spell radiating from the caster, so excluding him from the effect.
Spells with multiple and random effects:
You should be able to assign a "multiple choice" construct of spell effects, so there would be a chance of 10% for this effect and 90% of the other.
This allows for instance spells with a small chance to backfire.
In addition to that, more than one spell effect could be added, with a % chance of occurrence. If they are not part of the multiple choice construct, all of them can fire on the same cast.
These effects need to be able to have different targets. Some may fire at an area, others backfire on the caster himself or do yet another thing like cast a global tactical enchantment or change the ground under the caster's feet.
The ability to add (sometimes unwanted) side effects to spells would allow for far greater freedom in designing powerful spells.
Right now, if you design a powerful spell, it's just powerful. No reason for not using it...
Some units (like the Magma Elemental) would become unreasonably powerful if their intentionally designed weaknesses could be circumvented by buffs, such as increasing movement speed or health.
There should be a flag with the unit, making it immune to any and all beneficial spells like buffs, regen, teleport, and heals.
This allows the design of fun and overpowered units... with irremovable weaknesses.
In rare cases such as the Magma Elemental, it could still be "healed" with strategic fire attacks because of a negative Vulnerability.Fire. This would be a very inefficient and costly method of healing but creates the option of combining spells to greater effect.
Duration spells with variable or incremental effects:
For some books such as Death, instant nukes are a lame substitute for the whole sapping life force / fast ageing theme.
Some "1000 cuts" spell could be doing 3 damage on the first round, 4 on the next, then 5...
Alternatively: 1, 2, 4, 8, 16...
Additional twist: Save vs. Vulnerability.Death every turn to avoid the increase.
Such spells (especially with a 1,2,4,8... progression) would remain useful throughout the game even if unit HP rise from a mere 10 to 1200 or more.
Compare that with the Fireball as of 1.08: 11-14 damage + INT/10 + Shards*3
You would need 94 fire shards and 50 INT to even do a quarter damage to such a 1200 HP unit. Some fireball that is.
This mechanic does not necessarily need to be limited to plain damage. A debuff could sap 10% of a unit's current strength every turn. That would be most effective on very strong units but have dimnishing returns the weaker a unit gets. Again, that spell would be equally useful at every stage of the game.
Partial effects / resists:
Debuffs that have a constant effect such as "-1 combat speed for the duration" are okay but bland.
Some spells (especially debuffs) should be unresistable but their effects can be resisted.
Such as the "100 cuts" spell I mentioned above. As long as the unit keeps resisting every turn, the spell will never do more than 1 damage / turn. Pretty inconsequential.
But with a high vulnerability to that magic or plain old bad luck, it can get ugly, doing more damage every turn.
Same with debuffs. A "weak knees" spell should reduce the movement of a unit by 1.0 ... or 1.5 if it fails the saving throw for this round.
The all-or-nothing system is bland. Magic should work in fantastic ways, not like in a simple spreadsheet.
Irresistible spells with partial effect resists reduce micromanagement. There is no "bad cast" forcing you to recast it until you get full effect or any at all.
Yet the magic vulnerability of a unit always matters. You can have your cake and eat it, too!
Especially with debuffs you often get very little bang for the buck since making one unit fight a little worse is usually far less useful than buffing one own unit to fight a little better by the same amount.
The enemy unit is typically debuffed for one "encounter" in a battle while the buffed own unit (typically a strong unit to begin with) gets the benefit for multiple encounters.
There is also no possibility for the own unit to resist the effect.
Just like in many MMOs, the utility of debuffs is limited to fighting few but strong units. Something that hardly ever happens in Elemental...
Either debuffs have to be dirt cheap so they can be tossed around liberally (which needlessly increases micromanagement) or they must be irresistible / partial resist / AE debuffs, like some that I suggested for the Death Book.
What about focus items?
They can be researched / equipped like weapons / armor and alter the efficiency of a spellcaster. Less cost, more oomph... whatever.