God observes that the builders of Babel have one language, that “this they begin to do,” and that “nothing will be restrained from them, which they have imagined to do” (Gen 11:6). What does all this mean?

“This” presumably refers to the building activity. The things the Babelites “imagine” they could do would probably not extend beyond then-present experience; so we should not think that God is ascribing divine or even super-human abilities to the Babelites. He is, rather, concerned that they will further unify all mankind under the yoke of an idolatrous regime. As a famous psalm puts it, “Why do the heathen rage, and the people imagine a vain thing?” (Ps 2:1) As to having one language and dialect, this is a prerequisite of such a universal empire (as the lingua franca of Latin in the Roman Empire, or English in the present British-American Empire).