I know NWN did not make this easy at all, they hardcoded the spell lists for the various domains, put nothing in easy to change 2da files.

I thought it may be possible to do it with special INVENTORY items. which are given as you log in depending on your chosen spheres.

Not sure how scripting for spells goes, but casting a level 2 cleric spell on the ITEM itself would then allow you to use that item on a location.

You may have FIRE AND SUN as your spheres. When you log in it gives you FIRE DOMAIN object (for sun) and SUN DOMAIN OBJECT

when you cast a level 1 spell on the sun sphere object the object changes an internal variable to say that you're now able to cast colour spray ONCE with this object.

If you cast a level 2 spell, like cure moderate wounds on the FIRE SPHERE object, it changes the internal variable to allow you to cast COMBUST from the object.

The object is like a wand.. once you've expended the spell, the variable changes to say you can no longer use it for that spell again (until it's setup by casting a spell on it again)

I dont know how feasible this is, but it seemed like it may be a solution. Scripting people will have a much better idea. It's possible that the solution to this may only be possible with a hak of some sort which I'm sure you want to avoid.

Following is a list of spheres with some suggestions (included the un-implemented: chaos, law and luck) that use spells currently in game.

Air:  Electric Jolt(empowered)(1), Gedlee's Electric Loop(2) ,  Gust of Wind(3), Call Lightning (4), Chain Lightning (6), Storm of Vengeance (8)
Animal:  Hold Animal(1), Cat's Grace (2), Dominate Animal (3), Polymorph Self (5), Shapechange (9)
--> Chaos:  Daze(1), Tasha's Laughter (2),  Cloud of Bewilderment (3),   Confusion(4),    Mind Fog(5)  , Weird(9)
Death: Stone Bones(1), Death Armor(2), Phantasmal Killer (4), Enervation (5) Finger of Death (6), Wail of the Banshee(9)
Destruction: Horizkaul's Boom (1) , Contagion (2), Stinking Cloud (3), Crumble (5), Bombardment(8) 
Earth: Grease (1){mud}, One with the Land(2)[druid], Stoneskin (4), StoneHold (6), Earthquake (7).
Evil: Negative Energy Ray (1), Darkness (2),  Negative Energy Burst (3), Enervation (4), Create Undead (5), Aura Vs Alignment(7)
Fire: Flame Lash[druid] (1), Combust(2), Wall of Fire (3), Elemental Shield (4), Inferno[druid] (5), Firestorm (7).
-->Law:   Hold Person(1),   Clarity(2) ,  Hold Monster (4), Bigby's Interposing Hand (5), Power Word Stun (7)
-->Luck: (1) Entropic Shield (empowered) , (2) Aid (empowered) , Freedom of Movment(3), Battletide (4), Protection From Spells(7)
Good: Aura of Glory (1), Negative Plane Protection(2), Death Ward(3), Lesser Planar Binding (4), Banishment(5), Aura vs Alignment(7)
Healing: Lesser Restoration(1), Cure Serious Wounds (2), Healing Circle(4), Heal(5), Regenerate[druid] (6).
Knowledge: Identify (1), Knock (2), Clairvoyance/Clairaudience (3), True Seeing (4), Legend Lore (6), Premonition (8). 
Magic: Lesser Dispel (1), Dispel Magic(2), Minor Globe Invul (3), Lesser Spell Breach (4), Lesser Spell Mantle (5), Spell Mantle (7), Disjunction (8)
Plant: Entangle (1), Barkskin (2), Spike Growth[druid] (3), Mass Camoflauge (4), Vine Mine[druid] (5), Creeping Doom (7)
Protection: Shield (1), Protection from Elements (2), Globe of Invulnerability (4), Energy Buffer (5), Ethereal Visage (7) 
Strength: Blood Frenzy[druid] (1), Bull Str(Empowered)(2), Divine Power (3), Stoneskin (4), Bigby's Grasping Hand (7), Bigby's Crushing Hand(9)
Sun: Colour Spray(1), Searing Light (2)  , Hammer of the Gods (3),  Sunbeam (6), Prismatic Spray(7), Sunburst (8). 
Travel: Expedious Retreat (1), Glyph of Warding (2), Freedom of Movement (3), Slow (4), Haste (5), Mass Haste (7), Time Stop (9)
Trickery: Camoflague[druid](1), Invisibility (2),  Displacement (3), Invis Sphere (4), Improved Invisibility (5)
Water:  Ice Dagger (1),  Melf's Acid Arrow(2),   Poison (3),  Ice Storm (5),   Drown[druid] (6), Horrid Wilting (8)
War: True Strike (1),   Cat's Grace(2),  Keen Edge (3),   War Cry[bard] (5),   Aura Of Vitality (7)

It would also be nice if polymorph self and shapechange (in the animal sphere) were overriden to only give animal forms, and not monsters and golems etc.

That would make clerics Even more powerful, opening their rather large spell inventory from Alot of buffs and offensive spell/buffs to a decent ammount of mage spells too, so they can tank, and cast the hell out of the enemy even more so than before.

Oh and they wear plate and carry shields.

I vote no. Sorry.

Good heavens (pun intended) don't give more cheese to the Clerics!!1

They are reliable and versatile as it is. Powerful combination.

Let's not have clerics running around with premonition and casting timestop, please.

I don't think this is a terribly good idea.

Ahw, can't we just implement the War Domain?

War Domain True Strike (1) Cat's Grace(2) Keen Edge (3) War Cry[bard] Aura Of Vitality (7)

Clerics are the roughest and toughest people around. Do -not- make them more powerful.

Freaking powerhouses.

Clerics are really the most powerful class in the game as is, this would take a huge ammount of coding that could be better used on other projects, and this would bloat the module. Not going to happen.