Here's a list of spells that you currently get going down each path (and my comments on them), gotten from data file digging:
Air 1:
Evade (has been nerfed hard, but still okay to use on core units/champions)
Haste (great buff)
Air 2:
Aura of Grace (great city enchantment - units with more initiative is bad? come on...)
Guardian Wind (too situational, and archers aren't that powerful that you'd need it)
Gentle Rain (also requires Water 2) (great city enchantment - more food = more people, more tax, more research, more production)
Battle Cry (also requires Life 2) (a decent spell, but I don't use it much)
Contagion (also requires Death 2) (very powerful poison spell)
Air 3:
Storm (a meh spell... I would only use if you don't have fire or death)
Thunderstorm (it's an okay teleport to the front line kind of skill... I only use it when I can't move there fast or something)
Air 4:
Cloud Walk (godly)
Titan's Breath (broken? doesn't seem to knock down anyone at all.)
Air 5:
Celerity (decent, but too mana consuming...)
Tornado (it's okay... wouldn't be my first choice on a strategic spell, but if you've got nothing else...)
Earth 1:
Nature's Cloak (good resistance spell, but not used alot until very late when units starts fielding lots of magical weapons... or against fire elementals)
Enchanted Hammer (great city enchantment - faster production? yes please)
Raise and Lower land (situational, but handy sometimes)
Earth 2:
Stoneskin (good defensive spell)
Tremor (sometimes even more useful than a pillar of fire... it gives you two turns to bring your army back to beat it down... can stop a drake from leveling your city, where as pillar might not)
Sunder (also requires Fire2) (too situational... and sometimes doesn't work?)
Stinking Mud (also requires Water 2) (one of those odd battlefield enchantment - I don't use it much)
Nature's Call (also requires Life 2) (could be useful early, I don't use it much though)
Syphon Strength (also requires Death 2) (situational, not used often as a result)
Earth 3:
Create Mountain (expensive situational, but shutting down your empire is quite useful)
Shockwave (costly, but okay for an aoe touch range spell)
Earth 4:
Destroy Land (cheaper than Create Mountain)
Summon Earth Elemental (a good summon)
Fracture (only useful on super high HP units... and there aren't too many, so not used much)
Earth 5:
Earthquake (would be more useful if you couldn't take and destroy the city so easily)
Giantform (pretty godly for noncaster champions)
Birth of Summer (needs the research) (expensive terraform spell to turn arid land into fertile ones, too hard to get)
Bloom of Twilight (needs the research) (expensive terraform spell to turn arid land into fertile ones, too hard to get)
Fire 1:
Burning Hands (eh, has to start somewhere... right?)
Pillar of Flame (good overland spell)
Candlecloak (needs special ability) (a weaker mantle of fire for all your units)
Protection for Fire (needs the research) (okay, not used much)
Fire 2:
Burning Blade (great enchantment for heroes only)
Flame Dart (Bread and butter?)
Sunder (also requires Earth2) (too situational... and sometimes doesn't work?)
Devotion (also requires Life 2) (a decent unrest reduction enchantment, you pay the mana over time instead of upfront like bless city)
Soul burning (also requires Death 2) (Wickedly powerful tactical direct damage spell)
Fire 3:
Focus (better than wait)
Fireball (Boom!)
Feedback (a resistable counterspell that hurts the enemy caster if you succeed... I don't counterspell)
Fire 4:
Summon Fire Elemental (decent summon)
Mantle of Fire (okay, but barely ever used)
Fire 5:
Firestorm (been avoiding it 'cause of the remaining fire bug... not sure if they fixed it)
Wall of Fire (would be more useful if the AI ever makes it to my cities... I like my outposts and improvements too much to turtle behind the city walls)
Water 1:
Mediation (I don't use it much, but it can be handy for mana... particularly if you spam cities)
Slow (good debuff, can be completely crippling with enough water shards)
Protection from Cold (needs the research) (okay, not used often)
Water 2:
Chaos (I'm not even sure what this is suppose to do... most of the time it just fizzles? No idea.)
Freeze (an okay army debuff to use overland...)
Gentle Rain (also requires Air 2) (great city enchantment - more food = more people, more tax, more research, more production)
Stinking Mud (also requires Earth 2) (one of those odd battlefield enchantment - I don't use it much)
Mantle of Oceans (also requires Life 2) (Really useful enchantment. No maint and reduce spell cost? Yes please.)
Horrific Wail (also requires Death 2) (meh... does double the caster level to all units, but takes a while to cast)
Water 3:
Pandemonium (It's an okay spell, but the effects are too random to be used effectively)
Summon Ice Elemental (Okay summon)
Spell leech (a resistable counterspell that gives you 40 mana if it succeed... I don't counterspell)
Water 4:
Blizzard (not quite as good as fireball if you have lots of shards... but still, a good spell... no boom though)
Tidal Wave (it's kind of fun... oh, and it doesn't say, but it does [(3 x water shard) + 3] x 3 the number of troops in target army...)
Water 5:
Grip of Winter (would be more useful if it was worthwhile against the AI... right now though... not needed)
Vetrar's Howl (OW! No really, OW! It's like a super pillar of flame... costs quite super too)
Life 1:
Courage (decent, extra hp is always good, spell res is nice too... sometimes)
Regeneration (good spell early on, even better if you abuse it to heal by running around)
Life 2:
Aura of Vitality (great city enchantment... more HP on units produced... that almost feels like OP)
Heal (I think it's good, but apparently there are some who thinks it's OP)
Battle Cry (also requires Air 2) (a decent spell, but I don't use it much)
Nature's Call (also requires Earth 2) (could be useful early, I don't use it much though)
Devotion (also requires Fire 2) (a decent unrest reduction enchantment, you pay the mana over time instead of upfront like bless city)
Mantle of Oceans (also requires water 2) (Really useful enchantment. No maint and reduce spell cost? Yes please.)
Life 3:
Growth (situational, useful most of the times)
Shrink (good on those strong mobs if you can land it)
Life 4:
Call to Arms (40 Mana for a unit that might cost thousands of gold to rush? ... )
Wellspring (now THIS is an OP healing spell. Jeez... complain about the right one.)
Life 5:
Crusade (OP - seriously, 1 level for ALL your units... toss on some +per level traits, weapon and champion gear... *sigh* I cast it just about every time the cooldown is off.)
Death Ward (not too useful by the time you get it, because your stack of doom tend to never die at that point anyways)
Death 1:
Berserk (I don't like it much, but it can be powerful in the right situations)
Wither (godly... expecially since you can cast it twice... once on strategic and one in tactical... and they'd stack)
Death 2:
Blindness (a nice debuff, often makes powerful creatures useless if it lands)
Curse (also a nice debuff... if it lands)
Corruption (needs special ability) (transform shards into death shard)
Contagion (also requires Air 2) (very powerful poison spell)
Syphon Strength (also requires Earth 2) (situational, not used often as a result)
Soul burning (also requires Fire 2) (Wickedly powerful tactical direct damage spell)
Horrific Wail (also requires Water 2) (meh... does double the caster level to all units, but takes a while to cast)
Death 3:
Antipathy (Bleh spell, increasing unrest in opponent cities? w/e just take and destroy it)
Drain Life (good damage + self healing)
Graveseal (needs special ability) (completely OP, just as its name says, you're about to die if you don't resist.)
Death 4:
Mass Curse (curse everyone... just more costly mana wise... depends on the situation)
Touch of Entropy (powerful touch range spell)
Dirge of Ceresa (needs special ability) (Contagion + Front side damage makes it really powerful)
Death 5:
Blood Rage (decent, but again, too mana consuming...)
Sacrifice (good if you're going to raze the city anyways)
Not sure why there's no Fire 2 Air 2 spell, but I think they just didn't put in a requirement for Falling Star (from Third Book of the Magi). Hope that helps. To be honest, I think they did fairly well with the list. While some aren't quite as useful as others, if you don't have access to others, or if you have more shards ... etc, they might be worth it.