Druid is known to be the most versatile and most complete class in terms of resources. When I think about a druid I see a class that have most of the classes resources but in a less powerful version of them, and to balance that the skills are restricted by transformation and weapon used. I think druid must have 1 transformation per role and type of damage, for example, werewolf: melee dps as warrior or assassin; owl: healer; hippopotamus: tank; plant man: a nature caster. But the strong part of the druid is that can use all of his resources from different classes but shapeshifting.
In terms of specialization it should have a specialization per transformation to empower that form.
In terms of skills it should have every type of skills of whole game, but a less powerful version. So if we take a buff from the warrior, this buff used by the druid must have less duration, less stats increments and must be in werewolf (warrior melee dps form) to cast it.