Godzilla and friends

[Ronin]

Cool stuff, toyhandle! Gonna share my thoughts starting from the Imago form and working backwards. (Mostly my thoughts on the Shobijin and the Egg just follow from my thoughts on the Imago/Larva)

Mothra (Imago)
I'm a little iffy on Deity as a class in general, but it's in the toolbox, so what the hey. Agreed that it's a solid fit on Mothra.

Personally I don't like her being Kaiju Professor X. That version of QotM does really evoke Ghidorah so it doesn't doesn't feel wrong, but it's not must-have either, and I think it really pigeonholes her, in terms of army builds. That was something my first draft got flak for, admittedly, but I think this one hits the all-Kaiju-huge-game-exclusivity way harder; Mothra's like the Superman or Batman of her universe and I really want her to be someone I can team up with other icons, y'know?

I thought about doing Rebirth stuff but deliberately avoided it for basically the same reason: I wanted to keep the Imago form as self-contained/crossover-friendly as possible, and leave the life cycle stuff up to larva/egg cards to carry.

Mothra (Larva)
I think you're hitting all the right theme notes in this write-up, but the implementation of the Coccoon is really involved. I'd simplify way down to:

COCOON
At the start of any round, before placing Order Markers, Mothra may Coccoon. For this round, Mothra cannot move, cannot attack, cannot use its Kaiju Coordination special power and adds {#} to its Defense number. At the end of the round, if this Mothra is not destroyed, replace it with a previously destroyed Event Mothra you control. Remove all Wound Markers from that Event Mothra's Army Card.

I think the cocoon taking a little time feels more thematic, and having the window where it can be attacked counter-balances the ability to keep spawning Imago Mothra into the game.

I also probably wouldn't go full-on Queen of the Monsters on the Larva form; irrespective of what that power ends up looking like, I think it cheapens it a little if the Larva also get it. I want that Mothra transformation to feel like a big deal.

I do like having bonding on the Larva though. I'd just definitely give them a separate (probably simpler) power. Something more on the level of like:

After revealing an Order Marker on this card and taking a turn with Mothra, you may take a turn with any other Mothra you control, or a Kaiju you control within 6 clear sight spaces of Mothra. You may not take additional turns.

On the SA, I don't think it feels right for the Silk to be cutting through defense - I don't think of it as something that actually really does damage ever. (At least against the other monsters) I might not even implement it as an attack. Probably just a "choose a figure and roll a d20" style thing. It'd be cool if it had some lasting effect for the rest of your player turn that the bonding power could exploit. It'd give you an incentive to have them running around and actually doing stuff instead of just being Mothra resurrection batteries.

Mothra Egg
I was really debating the other day whether or not the egg deserved its own card; psyched to see someone else is this crazy too.

Like with the Larva, though, I'm a little leery of the complexity of the Life Cycle Marker stuff. And I'm not sure there's really a benefit to having anything hatch out of the Egg at present. It's just delaying the arrival of key figures to the game. I'd:
-keep the general concept of Undisturbed Nest
-let the Egg start wherever on the battlefield
-do like a The Drop style mechanic where eventually it just hatches. Maybe have the odds ratchet up over time.

That way you're at least getting a free deploy out of it.

Mechanically I don't think the "you control the Egg" stuff is important. I'd make sure to clarify that the Larvae occupy the Egg though - occupied DOs get some exemptions from auto-destroy stuff.

Shobijin
I like the concept of them helping Mothra integrate with more figures. I wouldn't do the attack boost thing, though - I worry that it incentivizes getting them killed. (And again, I'd really ditch the Life Cycle Markers)