Galvatron plans to summon Megatron to assist him, but it's not clear which Megatron he wants: the The Transformers (1984) Megatron or the Beast Wars: Transformers (1996) Megatron. Given the appearance of Optimus Primal from Beast Wars, it's likely he wanted the latter version.
This is the second full-length animated "Transformers" film to release in the franchise's entire history.