Unfortunately the Lycanthrope template's approach of "just add the Hit Dice together" inescapably ends up with creatures with excessive HD when a high-HD giant is combined with a high-HD base "beast". Standard lycanthropes can be extremely meaty such as wereelephants and some Dire Animal Lycanthropes (a dire werehippo giant can have ridiculous HD
). Giving them Dragon Hit Dice instead obviously just makes it more extreme. Imagine how many Hit Dice and Hit Points a Mountain Giant Drakanthrope could have!
I guess we should consider the base question as to whether a Weredragon should have higher Hit Dice than a True Dragon of equivalent size?
While a person supernaturally bonded to an animal might be a lot tougher than the base animal à la the SRD Lycanthrope Template, it doesn't mean a Humanoid (or a Giant) merged with a Dragon need be nastier than the base dragon.
Hmm, the original Weredragons were 8 HD man-dragon hybrids, and the text says the hybrid forms are "are halfway between the human and animal sizes". Taking that literally and assuming a "6' base" for a typical humanoid, that suggests a White Dragon (24' base length) version of a Weredragon is about fifteen feet long (24+6 divided by 2). Comparing all ten standard dragons, the Weredragons are all roughly the size of a "Very Young" 2E AD&D Dragon, which have Hit Dice four less than the base HD.
The breakdown is as follows:
White (24' base) => 15' long weredragon, 7 HD very young dragon
Black (30' base) => 18' long weredragon, 8 HD very young dragon
Green (36' base) => 21' long weredragon, 9 HD very young dragon
Blue (42' base) => 24' long weredragon, 10 HD very young dragon
Red (48' base) => 27' long weredragon, 11 HD very young dragon
Copper (30' base) => 18' long weredragon, 8 HD very young dragon
Brass (36' base) => 21' long weredragon, 9 HD very young dragon
Bronze (42' base) => 24' long weredragon, 10 HD very young dragon
Silver (48' base) => 27' long weredragon, 11 HD very young dragon
Gold (54' base) => 30' long weredragon, 12 HD very young dragon
Very Young is the second age category in 2E as it is in 3E. The smallest is "hatchling" that has 2 fewer Hit Dice (so between 5 Hit Dice for a White Hatchling and 10 Hit Dice for a Gold), which means the 8 HD of the Weredragon is closest to the 7½ average HD of the 2E Hatchlings.
Overall I think that supports using the Wyrmling age category as the baseline for our Drakanthropes, at least the human-sized versions.
After some reflection, I'm thinking we should consider making the Were-versions of the dragon significantly weaker than the genuine article rather than significantly stronger like lycanthropes are. The original has lower Hit Dice and less damaging physical attacks (i.e. 1d6 for claws and bite, while even a Hatchling 2E dragon does 1d6+1 to 1d10+1 with its claws and 2d8+1 to 6d6+1 with its bite). Plus they don't have the spell use or % magic resistance of the true dragons.
That'd be hard to combine with a generic template that includes standard monster dragons though.