In Exalted, Lunars are meant to be shapeshifters, but it always seems like the system doesn’t quite figure out what to do with that. The Tell kept it from being a perfect disguise, without Charms, but it still feels like a giant collection of Charms thrown onto the character for free, right off the bat. Just for being a starting character Lunar, one can fly, breathe water, generate any number of poisons, see in the dark, use sonar, disguise themselves from most targets, gain immense strength, fit into small places, resist heat or cold or lack of water, gain fangs or claws or horns, etc. And that is just the mundane animal kingdom. No, really, Etc!
Possibly an interesting system to investigate might be one where Lunars cannot shapechange into things which give them superhuman powers or disguise who they are without related Charms/knacks/whatever. Thus, a Lunar might not take on the form of a bird that can fly until she has a power that lets her fly – the learning of that power unlocks the flying capability of shapeshifting.
This could be done in a couple of different ways. You could let Lunars gain shapes of birds, etc, but be unable to actually fly without the required Charm. This would let them gather forms when the opportunity presented itself, and let them use them immediately, without giving the character the superhuman powers of every shape in and out of Creation. In a sense, it would also mimic the Tell situation a bit more. You can take these forms, but you cannot use them to their full ability. However, it might lend itself to some weirdness. Why is that fish drowning? Oh, it must be a Lunar who doesn’t actually have the water-breathing Charm.
Alternately, a Lunar might simple be unable to gain a new form at all if the form has abilities which they have not unlocked via some Charm/knack/whatever. A Lunar who does not have the flying power unlocked might be unable to gain the form of any bird that can fly. This isn’t as simple as it appears, however. Many animals, for example, can do multiple things that are beyond human ability. The Storyteller and players may not even know about some of these at the time when a form is initially approved.
My favorite aspect of the 2nd edition Exalted “knack” system was just that it eliminated the need to repeat the phrase “this Charm cannot be learned by Eclipse or Moonshadow” Caste Exalts. That could just as easily be accomplished by a Keyword, though. A keyword such as “Lunar-Only” or “Natural” or whatever could mean “you cannot get this Charm unless you are a Lunar” and that would eliminate much of what I think was gained by Knacks.
There is one other side effect of Knacks, though, and that is the fact you can give Lunars a number of them separately from their Charm count. This may be a good thing or a bad thing, depending on your view of it. If you want the possibility of Lunars who are just not as good at shapechanging as other Lunars, and can spend their time learning other Charms, then Knacks somewhat limit this. Sure you can stop learning more of them in the future if you want, but you are stuck spending some of your “starting” powers on them. If you want the possibility of Lunars who intentionally focus on other things Lunars can do, then you maximize this by merging Knacks and Charms into the same power list. If you want Lunars to always have some baseline of shapeshifting that they are required to take, then you possibly maximize this by having a category of shapeshifting powers set aside, such as Knacks does.
From a viewpoint of trying to examine the total Lunar package, though, it might be easier to merge Knacks into Charms. If you can readily see where Lunars get access to flight, while you are writing up other Charms, then you might have an easier time painting the quicksilver image of their power trees. It would also potentially let certain shapeshifting powers be tied to other abilities. For example, possibly before being able to shift into the form of a god, you might need the ability to sense spirits in their dematerialized form. To fully use the form of a god, you might need the ability to dematerialize. Alternately, being able to dematerialize at all might be a Charm which had a god/elemental form Charm as a prerequisite.