I don't think this this would be possible (or balanced) under the 4E desgin philosophy, but it'd be kinda cool if druid could be a multi-role character.
In humanoid form he would be a controller with all his spells.
He could then shapechange into a:
-big brute beast as a bear or gorilla to be a defender,
-big stealthy cat or raptor to be a striker,
-an alpha wolf or something like that to be a leader,
-tree to gain access to special healing and primal rituals, like communie with nature, etc.
For each form, the stats are the same no matter what creature the druid shapechanges into. In striker form, the druid gain a bonus to attack, damage and some rogue-like powers. In defender form he gains a bonus to HP, Defenses and some fighter like powers. And so on. While in any form he can't cast any druid controller spells, of course.
Shapechange could be:
-a minor action encounter power that last for the encounter duration or 5 minutes.
-a standard action at-will power that last as long as the druid wishes, but it is a standards action to enter any form and also a standard action to leave them.
I think the more abstract approach to shapechanging is what we are gonna see in 4e, even if there isn't this multi-role aspect.