It is indeed a general principle with powers that they end at the end of the encounter. There are a few explicit exceptions, and some powers that it would make sense to use outside of an encounter, in which case they work for 5 minutes (again unless there is some explicit text saying otherwise).
Outside of combat or for long duration effects you pretty much have to go with rituals. Aside from a very few high level utility powers that is really the only way to get long lasting effects.
So why wouldn't the best solution be a ritual? Especially when we already have the Tree Shape ritual to use as an example. It seems nearly perfect, just up the level to an appropriate level and make it allow various shapes. Call it "Wild Shape" or "Shape of the Wild" or whatever. You could make a series of such rituals for different shapes, but really it seems easier to simply make it use a Nature check result to determine what shapes are available when you cast it. If the check isn't good enough to get the shape you really wanted, then it is functionally a check failure, though you would have the choice to become a squirrel even though you wanted to be a raven ideally. The definition of success really being up to the caster.
At high levels the ritual, or a variation of it, could allow transforming multiple characters. There could also be versions with somewhat less strictures placed on it, like ability to take damage without dispelling the ritual, etc.
The casting cost can be relatively low, it is really in essence just an extension of the character's existing powers.
It could work something like:
Wild Shape
You assume the form of a creature of the wilderness.
Level: 6
Category: Exploration
Time: 10 minutes
Duration: 6 hours
Component Cost: 50gp
Market Price: 360gp
Key Skill: Nature
You transform into the shape of a natural beast. While you are in this state you are indistinguishable from a normal natural beast to ordinary senses. You retain all of your statistics, but you cannot use any of your powers or perform rituals. All your clothing and gear transforms along with you. The ritual ends and you revert to your normal form if you take any damage or take action, or at the end of the ritual's duration.
You must make a Nature check in order to determine the type of form which you can assume. After making the check you may choose any form allowed by your success roll.
[pre]
Nature Check Result Allowed Forms
9 or lower Medium or Small size
10-19 Tiny size, swim, climb, or terrain walk at speed 6
20-29 Fly, speed 8
30+ Large or Huge size, Hover
[/pre]
This probably needs a bit of editing, but it should give you a pretty good idea of where I'm going with this. Perhaps animal types could also be related to the check roll, so low level casters could mostly be just mammals, while higher levels would get you into birds, fish, insects, etc.
Comments?