At the moment my guess is it will be the candidate who can take Trump's broad brush on the issues and paint in the details. I think the candidate most likely to do this is Ted Cruz.
He is almost evangelical in his delivery. He is passionate and articulate. He also hasn't insulted Trump. If he can say, "Trump is correct and here's why..." I think it would carry him a long way.
At the moment I'm leaning toward a candidate who probably has no chance because the media is giving her short shrift. Almost to the point of being chauvinist. If only they felt the same about Hillary! What I like about her is her business background and understanding of how businesses are run. Exactly what our government needs - to be run like a business. As it is now it isn't being run at all. It's Camp Run-a-Muck. And like a poorly run company, doomed.
Realistically, however, I figure a politician is going to win the nod and it will be a matter of personality. Not a good reason to be sure but with 16 candidates running personality is going to be a large part of what helps a candidate break out. I think Cruz has that zeal.
They all are saying pretty much the same thing regarding policy. So far I haven't heard a one of them explain exactly how they are going to achieve what they all agree needs to be done. Especially if they don't have a solid majority in both houses. Actually they are a pretty boring bunch.
Personality could push some one past Hillary too. Hers is grating and fraught with ums and ahs. That is if she even succeeds. With the rumors about Biden thinking seriously about jumping in and Warren reconsidering, she could have some real competition. Webb, so far, has been a disappointment.
Of course my ideas change on a nearly daily basis as the jockeying for position continues. Trump, on the Republican side, does, like it or not, have a huge amount to do with how things shake out.
Even if Trump goes 3rd party I believe a strong Republican can prevail. Mainly because to date Trump hasn't said anything of substance either. I'm not sure there is any from which he can draw. Take note, Republican candidates. Get some personality and passion!