This is all about Godzilla, but when I hear that name I think of the whole universe full of characters. I'd want to see
someone from the old series; Anguirus, Rodan. And there definitely should be a new monster. Ushering in a fresh creature in the company of familiar faces is the best formula, I think.
And as much as King Ghidorah has been overused by Toho, I would also be thrilled at seeing him brought to life with big special effects (as long as he isn't too serpentine, like a stereotypical dragon). Too bad we probably
won't see him, since he'd cost Legendary more dollars. So presumably they have every right to use Angilas, Varan, Gigan, etc., right? Hell, they might as well throw in a Toho kaiju. They're paying for them anyway.
As for having other creatures potentially taking the spotlight away from Godzilla, I point to Godzilla 2000. To me, that was a perfect "comeback" movie, plain and simple. Godzilla, of course, rocked the house, fought the military,
and trounced on another monster. That's the way to go, I think. Actually, I'll probably be citing that movie a lot; from the suit design, the selective use of Ifukube (not too much or too little), G2000 perfectly exemplifies the type of "feel" Legendary should be going for. As long as:
1. Godzilla is not green 2. His breath is not orange.