The big thing to consider is; what will the Druid conceptually be?
 There is no guarantee it will be the stereotypical robed man with a long white beard down to his knees and a long wooden staff.
They may go in a totally different direction. Take the Warlock for example. If you asked me to picture what the warlock would be without seeing this game, I would not have pictured the lash of hellfire heavily armored battle mage image the game shows. I would have pictured a demon masked dark robed figure with a sacrificial dagger for example or someone similar to Ming the Merciless from Flash Gordon.
So that being said, the druid may be the full on elementalist or some other unthought-of of direction that we aren’t considering.

So if the Druid is going to be the traditional robed bearded old man standing in a stone circle then I picture they will be primarily Nature with a secondary choice of one of the four elements.
I think the Dark school will be forbidden for them.  
I think they will be more spell based in their special abilities since the Beast master is all about the minion summoning. I think the Druid will be about non creature spells and may even be a more solo mage like the Force master.

But if they do go nontraditional and the Druid was the elementalist of the game. My only concern about a dedicated elemental mage is that the elemental spells are primarily attack based. I would hope the new expansion comes out with more elemental creatures and non attack based elemental spells to help make the schools better rounded.

I am really excited to see which way they take this mage.