A couple of days ago, Movie Pilot's own Tyler Callaway reported evidence that Scott Lang will indeed become Giant-Man as part of Captain America: Civil War. Although, I admit that I'm sceptical. His evidence is fairly persuasive, but if he's right, then Marvel have made a mistake.
In Ant-Man, Scott Lang gained access to the Pym Particles, and used them to shrink to pint-size. As Scott was introduced to the technology, Hank Pym explained to him (and to the viewers) that he's stronger than normal when shrunken. The reason was an interesting one; because his molecules are more tightly-packed in shrunken form, they pack more of a punch.
The same Pym Particles can be used to both shrink and grow, and in Ant-Man we saw everything from a giant ant to a giant Thomas the Tank Engine! If Tyler's evidence is right, then in Captain America: Civil War Scott Lang will use the Pym Particles to grow in size, becoming Giant-Man.
Here's the catch: using the logic of Ant-Man, a giant-sized Scott Lang's molecules would be further apart. In other words, he'd be giant-sized - but weaker than a normal human being. No gigantic powerhouse - this Goliath would be a hundred-stone weakling!
The reality is, this is comic book logic, so there's always room for some creative hand-waving. But unless Marvel tread carefully, their technobabble explanation of the Pym Particles may have caused a bit of a continuity error...