Weredragons: These are people who are able to transform themselves into a hybrid form that combines a humanoid shape with distinctly dragon-like features such as scales, fangs, claws, wings, and a long tail. All have wings, even those species emulating the non-winged gold dragons. Each of these creatures have the same alignment as the dragon it resembles. They have 8 hit dice. Their bites and clawed hands each do 1d6 points of damage. They can also use the dragon’s breath weapon once each day.
Well that's not much to go on. There's also the common FR7 Lycanthrope "template" or rather "guidelines", and I quote:
Some guidelines for creating new lycanthropes are:
● Climate: Any, but tending toward the animal’s range.
● Frequency: Rare.
● Organization: As per normal animal.
● Active cycle: The lycanthropic form is only present during the night.
● Diet: As per animal.
● Intelligence: Any.
● Treasure: Any.
● Alignment: Any.
● Number appearing: As per animal.
● Armor class: Normal animal AC minus 1d4.
● Movement: Normal human or animal rate (depending on form).
● Hit Dice: Lycanthrope hit dice are the sum of normal human and normal animal hit dice.
● Number of attacks: As per animal.
● Damage/attack: At least as dangerous as the normal animal.
● Special attack: As per animal.
● Special defenses: 90% of new lycanthropic species can only be harmed by silver or magical weapons of +1 or greater.
● Magic resistance: 90% of new lycanthropic species have no magical resistance.
● Size: Any hybrid forms are halfway between the human and animal sizes.
● Morale: Most are steady (11-12) or very steady (13-14).
I'm assuming we'll use a similar approach to the Lycanthrope Template, only with a "base dragon" instead of a "base animal". I'm inclined not to give them a Curse of Lycanthropy bite since I don't fancy the idea of campaigns swimming in these critters.
So how close to the original do we want to be? Shall we keep the 8 HD and hybrid form only, or shake it up a bit by allowing full Dragon form and having the HD vary with type of dragon.
For reference, the youngest Small, Medium and Large SRD True Dragons are as follows:
Very Young Copper Dragon (Small, 8 HD)
Very Young Black Dragon (Small, 7 HD)
Very Young Brass Dragon (Small, 7 HD)
Very Young White Dragon (Small, 6 HD)
Wyrmling Silver Dragon (Small, 7 HD)
Wyrmling Blue Dragon (Small, 6 HD)
Wyrmling Bronze Dragon (Small, 6 HD)
Wyrmling Green Dragon (Small, 5 HD)
Young Copper Dragon (Medium, 11 HD)
Young Black Dragon (Medium, 10 HD)
Young Brass Dragon (Medium, 10 HD)
Young White Dragon (Medium, 9 HD)
Very Young Silver Dragon (Medium, 10 HD)
Very Young Blue Dragon (Medium, 9 HD)
Very Young Bronze Dragon (Medium, 9 HD)
Very Young Green Dragon (Medium, 8 HD)
Wyrmling Gold Dragon (Medium, 8 HD)
Wyrmling Red Dragon (Medium, 7 HD)
Young Adult Dragon (Large, 17 HD)
Young Adult Dragon (Large, 16 HD)
Young Adult Dragon (Large, 16 HD)
Young Adult White Dragon (Large, 15 HD)
Juvenile Silver Dragon (Large, 15 HD)
Juvenile Blue Dragon (Large, 14 HD)
Juvenile Bronze Dragon (Large, 14 HD)
Juvenile Green Dragon (Large, 13 HD)
Very Young Gold Dragon (Large, 11 HD)
Very Young Red Dragon (Large, 10 HD)
So it's the biggest species of True Dragon - the Red and Gold - that have the fewest Hit Dice when they reach Medium or Large, indeed they hatch out at Medium size.
The FR7 Weredragon's 8 Hit Dice does match two of the lowest-HD Medium Dragons, the Wyrmling Gold and Very Young Green, so we could use that as the baseline for our Weredragon's Hit Dice.
The averages are 6.5 Hit Dice for Small, 9.1 Hit Dice for Medium, 14.1 Hit Dice for Large, which suggests 5 or 6 levels of Dragon for Small Weredragons, 8 levels for Medium and 13 levels for Large.
We'll probably want to expand it to include Tiny and Huge or Larger Weredragons too.
If we have different types of Weredragons have different Hit Dice, it'd seem simpler to base it on age categories - i.e. a Small Weredragon has, say, a number of dragon levels equal to the Hit Dice of a Wyrmling Dragon, a Medium Weredragon uses a Very Young Dragon's Hit Dice, a Large Weredragon uses a Young Dragon's HD, a Huge one a Juvenile Dragon and so on up to a Colossal Weredragon having dragon levels equal to a Mature Dragon of its type (at +3 per age category that's 18 HD more than a Tiny Weredragon). All True Dragons can grow up to Gargantuan size, so if this template follows the "within one size category of the base creature’s size" rule of the Lycanthropy template all Weredragons will be able to go up to Colossal.
Don't really want a Colossal Weredragon to have 40+ dragon Hit Dice like a Colossal True Dragon, since it'll have plenty of giant Hit Dice from its base creature already.
'Course we could get around that by just having this template apply to Humanoids, but I quite like the idea of Giant Weredragons and the like. Presumably they tend to match the elemental type - i.e. Frost Giant White Weredragons.