Yes, this looks good!
Looking at the SAs, most of them would have a DC given by the Con modifier (for the base dragon), except for frightful presence (Cha). Should we say they use Con unless otherwise specified, and then note that frightful presence uses Cha?
I propose:
A weredrake retains the special attacks of the base creature or base dragon, depending on which form it is using. Saves have a DC of 10 + ½ weredrake's HD + the weredrake's modifier in whatever ability the form (creature or dragon) uses for that special attack, or Cha if this isn't noted.
In hybrid form a weredrake retains the special attacks of the base creature and gains the special attacks described below. If both base forms possess the same special attack, the hybrid form uses the base dragon's version.
Oh, and I'd tweak the Frightful Presence SA to "A weredrake whose base dragon possesses a breath weapon can use it when in hybrid form." As written it implies a weredrake in hybrid form has a breath weapon even if its base dragon doesn't!
Would you agree that we don't need any more SAs?
Yes!
Do we actually have any extra SQs to add? Since these aren't the true dragons, I'm not thinking of any off hand that necessarily fit enough of the rest of dragon-kind.
No, at the very least it needs the Alternate Form ability!
I also wouldn't be amiss to it being able to use some of its "draconic senses" and innate defenses in all its forms or its hybrid form.
Special Qualities
A weredrake retains all the special qualities of the base creature and the base dragon, and also gains those described below.
Alternate Form (Su): A weredrake can assume the form of a specific dragon (as indicated in its entry) or a bipedal hybrid of the base dragon and base creature.
Changing to or from dragon or hybrid form is a standard action.
A slain weredrake reverts to its humanoid form, although it remains dead. Separated body parts retain their draconic form, however.
Most weredrakes have full control over this power, but some are subject to involuntary change into their dragon or hybrid form (as per Lycanthropy as an Affliction in the Lycanthrope template) under specific triggering conditions. What counts as a triggering event varies between individual "afflicted" weredrakes, but common triggers are the sight of a large amount of treasure or the scent of a dragon and accumulating enough damage to reduce the weredrake's hit points by one-quarter (and again after each additional one-quarter lost).
Damage Reduction: The weredrake's hybrid form has any damage reduction possessed by the base dragon.
Draconic Senses: If the base dragon possesses darkvision, low-light vision or scent the weredrake can use these senses in any form. If the base dragon possesses other sensory abilities such as blindsight, it can use the special ability in humanoid or hybrid form, but when in humanoid form the ability has half the base dragon's range and/or effectiveness.
Draconic Defenses: A weredrake possesses the base dragon's immunities and resistances in all its forms. This usually includes the immunity to magic sleep effects and paralysis effects common to creatures with the Dragon type.
Drakanthropic Communication: In any form, a weredrake can communicate with creatures of their dragon form. If the base dragon is a bestial creature incapable of language, the weredrake can communicate simple concepts and (if the dragon is friendly) commands, such as "friend," "foe," "flee," and "attack." A weredrake in dragon form can always speak Draconic even if the base dragon cannot.
I think that about covers it! It seemed improper to give the weredrake full Drakanthropic Empathy with "+4 racial bonus on checks when influencing the dragon's attitude" so I trimmed it down to communication-only.
Oh, and should we say that weredrakes cannot be "dragons with age categories" as opposed to "true dragons"? I feel like linnorms and other age category dragons should either be weredragons or have their own version.
Yes, that sound good. How about:
The base dragon cannot be a dragon with age categories; drakanthropes based on such true dragons use the Weredragon template instead.
I usually think of all dragons with age categories as being "true dragons", including the Orientals and Linnorms.