Sorry for the necro
Sorry for another necro
See? Here's what I've said about necros taking up all the fun. Even werewolves' thread is overrun by them.
Both ideas are great by the way. Though with slaughter\fighting prolonging transformation time werecreature should probably have the ability to decide that there was enough havoc and to leave even if there's still dwarves to kill. (The idea of them running away before they turn into a naked humanoid among a dozen of angry steel clad midgets is probably good by itself.)
And the idea of them getting more powerful has potential too. They could become stronger\more resistant to damage\faster and so on. However there could be less hard-set conditions rather than "kill 3 hummies" and "kill vampire", they could gain new powers indeed by killing (yet gradually), or by virtue of being hereditary werewolf, or simply by with the time passing so that ancient ones could really inspire the respect for elder in young provided they don't die of old age.
On a second thought, "kill a vampire" doesn't even look too artificial to me. Some special condition for unlocking advanced abilities\progressing the curse (which of course can be randomized during the worldgen) isn't uncommon in myth: eating the heart of beloved ones, slaughtering some rare and magic creature, unicorn for example (not that they're really rare in df), transplanting eyes of another member of your clan...
Unfortunately, all this can and probably will be affected by personality (the amount of transformation time added, the probability of transformation, the curse progression for fulfilling conditions and whether or not creature will actively seek to fulfil them etc.), which means that they'll be in after the distant personality rewrite.