Walker followed Perry and by late November I'm sure many more Republicans will drop out. Trump will continue to have his 25% or so, but as more drop out, I think their supporters will migrate more toward Rubio and Fiorina. The establishment clearly wanted Bush, but Bush is simply not a good candidate. How long the establishment sticks with Bush I don't know, but his supporters will go to Rubio if things hold true.
If Rubio doesn't make any large blunders, I think he will be the nominee. Trumps is a loose cannon and as won't garner enough support needed to win the nomination when there are less candidates. Fiorina is one to watch, but she she have never lost the Senate bid to Boxer, so I would be surprised if she get's it. She is the media darling now though.
Don't be surprised if Trump is still doing well down the road that Romney doesn't get in it. When the field narrows, he'll have an even larger donor base to take on Trump.
On the Dem side, the party is clearly in it for Clinton, but if Biden get's in, he could be tough to beat (for the Republicans). Sanders has the far left crowd as a socialist, but would have no chance in a general election and the Dems won't let him be the nominee.