How Gigan can be brought into the MonsterVerse.

Gigan could be an alien Titan who comes from Planet X (which can serve not only as a nod to Invasion of the Astro Monster, but also reference real world conspiracy theories that involve Planet X/Nibiru), the home planet of the alien race (which could not only serve as a nod to the Xiliens from the Toho movies, but also the Anunnaki from the Nibiru conspiracy theory and Sumerian mythology). Before invading Earth, these aliens capture Gigan and conduct several experiments on him, turning him into the cybernetic monster we all know and love.

This would be a very interesting bit of world building as it could open the door for Monarch possibly exploring Planet X to discover new Titans in future films.