To be honest, DD is useless in NWN1 for casters. Except in a level 40 module, a lv 30 sorc/10 RDD isn`t that bad, but level up DD since you get no caster level like in 3.5e rules and the hit dice start with less than the rules, is useless. Note that spells that allow you to take dragon form, ~metallic~ dragons and everything is already in nwn1, so i don`t know why they din`t implemented dragon form and made this class useless for casters. If you pick a single sorc level only to level up a DD, his bonus is amazing for melee characters in nwn1.
Also, i don`t understand why they like "half implement" only fire/red, arcane archer can only imbue fireball(in rules they can imbue frost and electric spells). This of course in nwn. I never have played D&D 2e or read the books, so i can`t have an opinion about how BG DD implementation should be only that people who have dragonic lineage with metallic dragons will be far more common than people with a red dragon heritage...
For casters, dragon disciple is useless, sure. But given the +10 strength boost, it actually makes for a great melee build combined with a few levels of bard/sorc and some fighter levels. Bard 1/RDD 10/Fighter whatever's left is one of the most popular and effective melee builds in NWN.
As for why they only provide fire options for RDD or AA, I imagine it's to avoid having to put too much effort into it and for ease-of-use. Honestly, if they had had to implement every possible option for every prestige class they put in NWN, it would probably have taken far too much time.
Besides, the focus of NWN was the toolkit and I imagine they thought that if people really cared about silver dragon disciples, someone would make a hakpak for it.
When it comes to BG, though, the modding community is not nearly as large as it used to be and they tend to prefer the original versions to the enhanced editions, so modding a kit that's only present in the EE is probably low-priority.
Dragon form - Already in game(a epic druid can even cast dragon abilities)
Different breath weapon - Already in game
Different dragon types - Already in game
Caster level gain - Already in game(pale master give more CL)
Increase status when level up - Already in game
Resistance/Immunity when you level up - Already in game
Race change - Already in game(if you use shapechange to a golem, your race becomes construct in character sheet)
If they wanna put a fly mechanic, yes, will require some hard work to be implemented, but to put a DD that follows the rule(except by fly), you simple need to give stuff that already are in game. Imagine if you can only be a cleric of fire domain, even if the gains/spells/bonuses from other domains already are in game
... I will try find a mod to play with my favorite class in NWN and in BG
PS ? Sorry for discussing nwn in a bg forum.