Don't want to come off as a Godzilla snob or anything, but automatically thinking this'll be a "clusterfuck" definitely shows little understanding of the franchise lol.
I have seen a Godzilla movie every sunday for 11 weeks now this years, so I am aware that dropping randomly Monsters in to have them fight Godzilla works. I dont want a full Origin movie, in which we see Gidorah and Rodan hatch from a egg, grow to full size and then battle the King of the Monster, but I think, if you also want to do a proper Mothra movie, you have to do it right.
I have no idea how they are going to do it, since Mothras origin may be a bit to cheesy for a western audience, but I liked about Godzilla 2014 that it really embraced Godzilla and the japanese side of the character, so I would love to see them give Mothra the same amount of love and hope, they have something that resembles the Twins, the Mothrasong and the Infant Island. Due to avaiability in my region, I have only seen Giant Monster All Out Attack, but even this one had the Infant Island, the Twins and the song while cheating and relying on the original Mothra movie as already happened in this continuity. It can work, but I really dont want to see Mothra flying out of a hole in the earth and thats it.