We've seen the republican establishment have its feathers ruffled by a non-establishment right-wing movement (i.e., the tea party), and I think it would be great to have that same establishment shaken up a bit by a rogue non-establishment candidate from the center, such as Trump. I think I can fairly accurately predict that Trump would have positions and ideas all over the place (which I can admire), and that he would hold up pretty damn well in a debate.
And here's another thing: He would have to be taken seriously. He would have plenty of money to pour into his own campaign, and he wouldn't be running as some sort of Ross Perot third-party candidate. The best performance of any third-party presidential candidate in American history was the very charistmatic former president Teddy Roosevelt in 1912, and he was only able to get a paltry 27% of the popular vote.
Put another way: Third-party candidates don't win the presidency. Trump running in the republican field would be critical to him having any chance. So we'll see if he actually runs. And as indicated -- if he does, he might just be dangerous.