I bet Mothra will be in this minimally. I could see Godzilla mainly fighting one or two other monsters, with others just being in background or briefly shown.
Comments have been made numerous times from writing staff and the director, I think even Kong has shown the value of giving your main monster a prominent presence.
Pretty sure Rodan will steal the show. Movie will probably end with him killing off all the others and setting up "King Kong vs. Rodan" to replace the ill-conceived "King Kong vs. Godzilla" movie.
When I was a wee lad, Rodan was always my 3rd favorite monster (right behind Ghidorah and Godzilla) and now I don't know why -- he's just a silly looking bird after all. I think I made my ranking based on who was in 'Destroy All Monsters', that's why.