There is supposedly an article saying that a member at Toho was talking about Mechagodzilla's appearance in Godzilla vs Kong or in future films within the Monsterverse. If any body knows anything about the "truth" in this claim let me know. In the meantime I will take a look into the subject in question.
Topic on User talk:Toho Eizo Films
He cited Mechagodzilla as a popular character when asked about who he would want to see in future movies, and that's about it. Here's the interview: https://www.inverse.com/article/57875-godzilla-at-san-diego-comic-con-interview