[@Gutshot]

I like to think that Rose and Mothra could go on "isn't humanity neat" adventures together in human form a some point.
 
As for starts it either has to be something big enough that it drags in 5 Kaiju on top of the human defenders, like a full blown alien invasion, the end of evangelion because that an organisation in the rp now, which is cool, or meany meany monsters all show up at once in one place heading straight for some mcguffin. 

or slightly more silly some kind of social event that some how gets them together, like a Kaiju convention or a tea party at R'lyeh or diplomatic conference for Kaiju relations. That then gets crashed by more monsters.