If various sources are to be believed, we don't have to wait until tomorrow to learn Trump's VP pick: Mike Pence, Governor of Indiana. It would seem Trump decided to go with a solid, steady choice - a fellow with good experience both as legislator and executive who is also somewhat bland.
Given Trump's far-from-bland nature, perhaps Pence provides good balance. Trump is perfectly capable of generating himself all the campaign fireworks needed, and then some.
Later ... Folks have been pestering Trump to act more presidential, less the showman/huckster. The choice of Pence appears "presidential;" he's a serious fellow not given to flamboyance.