I have been reading Age of Treason and I really like the way the author has blended the magic styles. Common, divine, sorcery and spirit magic are not strictly independent of each other.
You could do something like this and it would make druids very different from standard divine magicians or sorcerors or shamans:
Druid Magic skill or maybe Druidic Tradition skill (this would serve as the Lore(druidic) for divine magics, Grimoire(druidic) for sorcery magics, common magic skill and spirit binding for spirit magics.
Druids might have a Pact(Nature) or a Pact with a particular god of nature.
Choose the spells you think are appropriate for druids (common, divine, sorcery) or make some up.
Common magic spells are generally less powerful for the MPs expended.
Divine spells are limited by the dedicated POW the druid has committed to nature, but require no MPs for casting. Pact(Nature) acts as normal for divine spells.
For sorcery spells, I think using the Pact(Nature) skill in place of the Manipulation skill would work. The druid's flexibility and control of these spells will increase with the strength of his commitment to nature or the god.
If you want to include some spirit magic, I like the Age of Treason spirit magic. The druid does not spirit walk, but either calls spirits to himself or seeks out locations where the spirits hang out in the material world. Spirits don't discorporate people into the spirit world, but rather manifest themselves into the material world. Then they can interact with corporeal beings. Spectral or spirit combat takes place in the material world. The druid may have a means of forcing a spirit to manifest, or maybe not. The Pact(Nature) skill could determine how many spirits the druid could control. Maybe 1 per 10% Pact(Nature) or 1 intensity of spirits per 10% of Pact(Nature). Maybe the number/intensity of spirits controlled reduces the number of manipulations available to druidic sorcery spells. For example, if they have 50% Pact(Nature) and are controlling 3 intensity 1 spirits, they would only have 2 points of manipulation available.
If you don't like idea of mixing all of these, then pick what you like.
For the animal companion, I like the idea of an animal spirit or nature spirit that is allied to the druid. The druid performs his ritual to summon the companion. The spirit possesses a suitable animal and comes to the druid. If you like, the druid and his companion could have a mental link which allows the druid to use the companion's magic points. You decide whether companions are animal spirits and always possess the same type of animal, or nature spirits that can possess different types of animals depending on what the druid called for in his ritual. The companion could possess larger or more powerful creatures based on the Pact(Nature) of the druid. Possibly the possessing spirit augments the size and strength (and/or DEX) of the creature, again based on the Pact(Nature) of the druid. If the animal dies or the druid dismisses it, the companion returns to the spirit plane (or where ever you want) and is out of contact with the druid until it is summoned again.
Sorry this isn't very organized or specific. It is sort of stream of consciousness. And it is late and I need to get to sleep.
