It might be a bit hard to do justice to Mothra in an American production, given that she's the kaiju who most embodies the spiritual/animistic underpinnings of the genre, more a deity than a monster. Not to mention the confusion created about the character by all the English dubs that called her "he" or "it."
Although I love Godzilla movies I really do not think many of Toho's other monsters would translate well for a mainstream audience. Most of them really do embody the Japanese culture.
While that may be true, I feel that aside from Godzilla, King Ghidorah is the most recognizable monster from Japan in the United States, even more so than Gamera.
He can just be used as he was originally, a terrible monster from space.