Apparently it's been confirmed: Mechagodzilla for the sequel. I think the images are throwing us off because we're expecting Mechagodzilla to have Godzilla-but-robot features, while this one does not and will resemble Godzilla more in shape and build. He reminds me of the Mechagodzilla designed by the Metal Gear artist for the Godzilla vs. Eva promotion:
Also, I saw on GMAN's twitter than an new article says this was initially pitched as a TV series, but Toho insisted on it being theatrical.