They have to call it something, and no one knows what the official name will be until Apple announces it. Your guess that it won't be numbered has no more supporting evidence than the guess that it will be called "8".
iPhone 8 or iPhone X seems very possible. "iPhone Pro" is not appropriately fitting at all for an iPhone. I don't see the logic behind an iPhone labeled "Pro."
Apple will likely name it with a new terminology after 'iPhone'. Akin to 'iPad Air'. This is marketing. You want to draw attention to new features/design when those changes are this great in scope.