Let us dispense with a few of the most ascribed to election dictums right off of the bat.
The vice presidential pick is rarely a game changer. When it is, it is almost always for the worst (see Palin, Sarah). Also, rid your mind of the need for so-called "geography balancing" picks; that hasn't really been a factor since Kennedy picked Johnson. Some may argue that Bush won Texas for Reagan, but that is thoroughly debatable.
The successful vice presidential nominees are the ones who fill an expertise gap. Bush 41 and Cheney covered foreign policy for their respective bosses, Gore covered legislative work from his time on Capitol Hill, and Biden covered both for Obama. Making the wrong pick can also doom a campaign (again, see Palin, Sarah), and the choices made to fill a geographic or demographic gap also tend to not help (just ask Joe Liberman, John Edwards, Dan Quayle).
Trying to predict anything Donald Trump will do or say is a fool's errand and in attempting to guess who he will convince to be his VP is even more thorny (plus, few may be interested). In all likelihood his short list likely includes the following: Chris Christie, Ben Carson, Newt Gingrich, Marco Rubio, Joni Ernst, Tom Cotton, and an unnamed retired military officer. Smart money is on Gingrich, Christie, or the retired military officer, but at the end of the day, Trump is just as likely to pick compulsively and erratically in typical form.
The "veepstakes" for Secretary Clinton, however, is truly intriguing. Between her time as first lady, senator and secretary of state, Hillary has the domestic and foreign policy spaces covered, so picking someone to fill and expertise gap is not needed. Another rule of vice presidential-picking is "do no harm." Picking a sitting senator from a state run by Republican governor can most easily do this, as they can appoint whomever they want to the seat and jeopardize the - potential - razor thin majority in the Senate. With this Hippocratic principle in mind, we need to eliminate Ohio's Rob Portman, New Jersey's Cory Booker and (for this and reasons explained below) Massachusetts' Elizabeth Warren.
Tim Kaine is a solid pick, and a smart one -- which is why his name has been in the mix for years. As a former mayor, governor, party chair, sitting senator on the Foreign Relations and Armed Services Committees, Catholic, fluent Spanish speaker, etc he is well-deserving of consideration. But let's take a step back and be more creative.
More than a little ink has been spilled about how the Democrats have a weak bench. While this too is debatable, having a wide open veepstakes makes it possible to bring new folks off the bench. One reason to do this is that should Hillary not win (she's currently a -300 favorite), a young(er) face could be available in 2020 (see Ryan, Paul). This is one of the reasons Secretary of Housing and Urban Development Julian Castro and his brother Representative Joaquin Castro (TX-20) receive attention. Although, it seems either are unlikely.
Despite what the progressive left might think, Elizabeth Warren is not an ideal running mate. In addition to giving up a Senate seat, combining Clinton and Warren could be like oil and vinegar. Besides--wouldn't you rather see Warren battling Ted Cruz in the senate for the next two decades? More importantly, whereas primary elections are always about the base, general elections turn towards the middle.
Sure, Warren could mollify the Bernie followers, but that is not the group Hillary should be concerned about grabbing. She needs to go after the moderate middle who find Trump reprehensible, and the Republicans who cannot imagine voting for Trump. Choosing Warren turns too hard to the left and makes it easy for those in the moderate middle (i.e. most of the country) to not vote for her. Finally, Trump has already "tagged" Warren--why give him more material? She's already proven in recent weeks that she's more valuable attacking from the sidelines.
The rationale undermining many additional nominees is the lack of national security credentials. The Republicans have already signaled that they intend to cut through the Trumpian cacophony by running their favorite, classic, tired and wrong strategy of "Democrats are weak on defense" campaign. Given the head of their ticket, that will be hard argument to make--but they're going to do it anyway.
Three extremely interesting choices would be South Bend Indiana mayor, Rhodes Scholar and Navy Reservist Pete Buttigieg (age 34), Massachusetts' congressman and Marine veteran Seth Moulton (age 37) and Hawaii's Tulsi Gabbard (age 35), who is also an Army Reservist. Any of these would ignite young Democrats and crush the "Democrats are weak on defense" line of attack. Sticking with that logic, Hillary can end that route of attack entirely by choosing a former high-ranking military officer.
According to reports, including Mark Landler's New York Times Magazine piece, Hillary developed strong relations with the military as a member of the Senate Armed Services Committee and as Secretary of State. General Stanley McChrystal (age 61) is highly respected, and enough time passed since a career-ending Rolling Stones story that his name should be towards the top of the list. Another special operations commanding officer - General William McCraven (age 61) could be an ideal candidate. The man who oversaw the mission to kill Osama bin Laden is well respected and has spent the last year as the Chancellor of the University of Texas System. An even more intriguing idea would be the first woman to ever achieve four-star officer status, General Ann Dunwoody (age 63).
These former officers may not currently be party activists, but with the prospects of a President Trump, they may not need much convincing--even General David Petraeus (age 63) may be interested in preventing Trump from winning.
At least since Mondale in 1976, the Office of the Vice Presidency has less resembled what John Nance Garner termed a "warm bucket of spit" and much more as the most senior advisor to the president. Biden asked that he be the "last person in the room", Cheney may have had too much power, and Gore was--at least for the first six years--a partner to Clinton. We've come to expect, as rightly we should, more than a pulse from our vice presidents.
But like so much else, the veepstakes comes down to political theater and manufactured intrigue. In the end, the only thing that should matter is if the person is capable of being a good governing partner and capable of being president should the worst happen. Anything else is just chatter.