Continents move – we know this. The Atlantic is growing thanks to the expansion of the Mid-Atlantic Ridge. But in the future the ridge may start subducting. And with it, the Atlantic may become an inland sea.
The possibility of an Atlantic sea is highly speculative. The idea relates to a hypothetical future united continent called Pangaea Ultima. (It would be ruled, presumably, by Lord British.)
Beginning around 124 million years from now, as the Mid-Atlantic Ridge begins subducting, North America collides with Africa. South America wraps around the bottom of Africa, and just like that the Atlantic is enclosed and becomes a sea.
This is just one possibility. Two other proposed future supercontinents (Amasia and Novopangaea) suggest the opposite: the enclosure of the Pacific and the growth of the Atlantic. Maybe I should have titled this one “Atlantic vs. Pacific: There Can Be Only One.”