The Godzilla Monster Planet sequel has been announced, Godzilla: Decisive Battle Mobile Breeder City (?), and it appears Mechagodzilla will be apart of it. The sequel, which also has a poster revealed as well, is scheduled for a May release and a ticket pre-order has already started with special early pre-order bonus of a clear file with the movie poster image on it (below)as well as the image on the ticket too it appears. The reveal was accompanied by some info (translated from Japanese) -
"What does "city" that was left on the earth after 20 thousand years that all urban institutions should have been devastated? Also, in the Tizaru visual, there is a scene that cuts out the moment that Humanity confronted <something> in the darkness where rubble was stacked up.
Whether the "mystery" that springs up in the movie "GODZILLA Monster Planet", the novel "GODZILLA Monster Revelation" is revealed. "GODZILLA Monster Planet" In the prehistoric period, what is "What is Mecha Godzilla", a godzilla decisive weapon secretly developed by mankind?".
More info as we get it on the sequel.