Whoever it is needs to be someone who is NOT a member of the political establishment. Having a non-establishment conservative VP on the ticket, someone with character provides some measure of insurance that the establishment will gain nothing by foul play against a president Trump.
Newt Gingrich is a moral zero, a man with zero apparent principles. If he is Trump’s VP pick, I will be supporting a third (ie. second) party presidential ticket.
I’d take the first part of that a bit further: I’d like to see Trump put his VP very visibly in charge of anti-corruption and “unrigging” the system. Put Guiliani in charge of the FBI (if he is up to it.) Christie as AG.
Sarah Palin probably has the energy and attitude to do the anti-corruption bit, but, I’m not so sure about the politics at this time. First, Trump has to get elected.
Cruz - well, there’s probably just too much bad blood.
Nikki Haley has the smarts, but, I’m not so sure about her Tea Party credentials, anymore.
Pence has some experience as an executive, but, is he seen as willing enough to buck the establishment?
Ernst I really don’t know enough about.
Newt is quite controversial even within conservative ranks.
But then, who? Hmmm...