When making a game about giant monster battlin', it's important to make sure that you can pretend to be the guys that you know you want to be. To that end, you've got to make sure you've got all the bases covered. You need to be able to emulate a good, old-fashioned Godzilla versus Mothra battle, and have more or less accurate attacks and stuff open to you. And since it boils down to the two of them brawling around for a while, well, that's actually kind of easy.
He only seems vanilla because we're used to the idea of a freakishly strong radioactive lizard with a crazy-ass breath weapon, but he's not exactly standard monster fare. He's shown as being tougher and stronger than pretty much any other monster, in addition to being able to kill pretty much anything. His only real weakness seems to be electricity, for some reason.
I think that Armored Hide is a pretty good ability- a couple of monsters have it, and they're noticably tougher than ones that don't. The same with his Beam Weapon breath- lots of monsters shoot stuff out of their mouths or eyes, and you can use it for secret anti-monster weapons, too. He seems to be very strong, so maybe Mighty would be appropriate- he certainly is a lot stronger than he seems like he should be.
Mothra is pretty much my personal favorite. I dunno, I just kind of like bugs. Anyways, Mothra can flap her wings and create massive gusts, fly around, and slam into stuff. Sometimes she can also shoot lightning or energy or whatever, but I don't know if that's something she does all the time or what. She doesn't have very useful arms, but she can still pick stuff up, so there is that. The rest of what she does (reincarnating, molting, having pop singers talk for her, psychic powers) aren't really relevant to the smackdown, so they're all omitted. Feel free to imagine them in, though.
Mothra is Flying, obviously. She has an electrical Beam Attack in a couple of incarnations, and I think that's always fun to have. She can Gust using her wings, which I think could be fun to include- you could have a monster who can breathe really hard, or use it for a lot of other Flying type monsters, but not every flyer bothers to make tornadoes.
Ghidorah is a funny chap. He's flying but he's kind of clumsy, and he's got a really strong breath attack, what with his three heads. He's not very good at aiming, and he's not very fast, but he's tough as nails and extremely dangerous. It normally takes two or three monsters teaming up to bring him down, which is pretty fun, especially since you're busting down buildings and maybe working together with a super secret military project. King Ghidora is probably the most powerful kaiju of them all, and really requires a lot of rule mangling to get him into play.
He's got Armored Skin and a really powerful breath weapon- maybe he's got three Breath Weapons, which he can use all at once. He's kind of slow and clumsy, and he gets shit done mostly through being big and strong rather than actually being good at aiming his lightning breath or smashing things intelligently. Like in Godzilla vs Monster X, Godzilla hides behind a rock while King G. stupidly blasts the thing. I know he's mind controlled but come on, big guy. So he's kind of Stupid. I don't know if it's worth it to have that be a thing, or just lower King G.'s Activation and Expertise. He's still Really Strong.
I dunno, I'll have to think about it some more. Guy's kind of a pain with his weird triple heads and wandering idiocy.
Flying: Flying monsters have their own wound chart. Flying monsters can move over terrain and monsters in their way, and don't have to land.*
Beam Attack: You can attack things at long range.
Gust: Everything in your line of sight has to roll against their Size or be pushed back 1d6". Failing by 2x Size means getting pushed back 2d6". Hitting something means taking at least a Str 1 hit.
*The wound chart would have entries related to damaged wings and such, so that being a flying monster means you're more vulnerable, but it also means you get to flap around and be a pain in the ass. You also get to smash into the ground and maybe make a bigass crater.
Up next time!
Mecha-Godzilla, Rodan, Gamera, Daimajin, Gappa, Guilala, and more!