Fire domain
When the spell Firewall (fire domain) is cast, the lvl 3 spell FIREBALL is cast instead, with the same progression as a wizard.
Reasoning: Even for RP purposes, the fire domain doesn't allow anyone to showcase any ability that other clerics don't have. Fire wall is particularly useless, low duration, low damage spell, while regular clerics gain the firestorm and flame strike anyways, both being particularly better and much more flashy.
Restrictions would make sure wizards continue to be kings of fireballs since wall of fire has a medium range and one slot higher, so clerics using this should be extra careful. Still, this makes for a much more atractive use of the fire domain, wich is usually repulsive to players. I mean, ther's no reason to take the domain since even if you WANT to be a fire cleric, you'll getgreat fire spellswithout it anyways. The reasoning here, is you should be rewarded with appropiate spells for taking a specialized domain, instead of it being "just for shows". MY take is Roleplay decisions should manifest into rewarding in-game effects and not just make-believe.
Water domain
When the Poison (or Poison, water domain) spell is cast, the monster ability POISON CONE is used instead.
The Dc is nothing to write home about, but it's still a grand improvement to the current lackluster poison experience for spells. If it was posible to use the "VErmin" poison cone, it would actually mean business tough, since max effects reach 35. It's hardly overpowered since we are talking about a spell that can be removed by a potion in a single round, and is much less hostile in a combat situation than say, Hold person. (reference, http://nwn.wikia.com/wiki/Poison_cone)
REasoning: THe "Poison spell" has yet to be seen in action. Not to mention that it's max DC is 18, it only removes 1d6 strength. Not your spell of choice for rat-catchers. I know there are rp purposes, but when you spend 3 months playing a lvl 15 cleric of Talona and can't do squat with your awsome poison powers... (At least if you were an awsome possum, but no, it's awsome poison). Other characters question your sanity and with good reason. You devote yourself to the godess of poison and get zero in return. it's really time to move on. On top of it all, clerics already get this spell at circle 4. So i propose the spell itself, domain and regular, are modified.
Evil domain
When Enervation (Evil Domain) spell is cast, Evil Blight spell is used instead. (http://nwn.wikia.com/wiki/Evil_blight)
This is just a personal prefference. Instead of Ennervation, a deliciously evil spell allowing you to curse ALL enemies is used instead.
Earth domain.
Now this is just a pet peeve for me
http://nwn.wikia.com/wiki/Hurl_rocks Grant one use of this skill per day to earth domain priests
