I will forever hate the responsible people for throwing away what could have been the most amazing storyline on this show. I love James Patrick Stuart, I do, but nobody in this wide world can tell me Season 7 couldn’t have been extraordinarily amazing with Misha playing the head of the Leviathans. The struggle the boys, especially Dean would’ve had to go through deciding whether or not to attack and possibly kill their friend, Castiel at times getting through, maybe in Dean’s dreams, maybe when facing the brothers, begging them to kill him. Oh how good this season could’ve been if only… if only……
PREACH IT.
To be fair, I don’t think the writers realized just how manic Misha was going to play Leviathan!Cas at the end of 7.01 when they wrote his little part. But he ended up taking that scene and playing it far beyond the expectations. Unfortunately the fact that he played it SO WELL meant the fact that we never saw any more of Leviathan!Cas was quite a comedown.
I’ve never quite understood why the Leviathan never again used Cas’ form to torment/mess with the Winchesters. It would have been fantastic. But they didn’t go that route for whatever reason and that’s that.
Season 7 is what it is. Much of it fell flat for me (although when it was good, it was GOOD) and felt rushed but it happened. It had good concepts poorly executed and I agree that it could have been so much more. However I’m kind of over complaining about it now that I am enjoying the arcs and character focus in season 8. But yeah, sometimes I do feel sad that season 7 never followed through with the epicness that 7.01 and 7.02 promised.
Honestly, they could have re-jigged the season when they found out how amazing Misha was as the Leviathan, and how everyone responded to him. LIke, you said, they could easily have had the Leviathans take his form again.

(take a closer look.)
I hereby challenge you all for the right to my vessel! You will leave it, twisted cretin, or suffer the full wrath of a warrior of heaven.look who’s up and about! you bounce back quick, kid, we’ll give you that. are you sure you’re ready for another round? because when we win, we are going to tear you apart and stitch you back together all upside down and backwards. it will be hilarious.
some of you have expressed an interest in meeting us face to face. being in the physical presence of a GOD has never been a healthy life choice for mortals, but we’re sure you’re the exception.
WOAHWOAHWOAH CLICK ON IT GUYS
I JUST ABOUT SCREAMED
AWWWWESOME
fLIES BACKWARDS
/SCREAMED AND FLAILED REALLY BAD

we’re too big for you to bully now, DADDY. you shouldn’t have locked us away like that. we’re going to swallow up these humans, your favored sons. and then we will come for YOU.

“Daddy’s hungry, children.”
I really do wish the Leviathans had stayed in Castiel’s body. They would never have taken Roman; why would they, when they already had what both heaven and hell regarded as the new king? Jimmy would deteriorate over time, sure, but they’d maintain him the same way Lucifer maintained his imperfect vessel: sheer, unbridled decadence. Then, instead of railing against faceless corporations, which is as frustratingly futile as it is boring, the Winchesters would’ve had to figure out how to kill a mad new god who had cowed both the host of heaven and the hounds of hell. Angels and demons and corporations all follow laws or prophesies, they can be predicted; but leviathans could have been an element of chaos, the part of free will that does what it wants because it can. (Not to mention Misha Collins could act the shit out of a ravenous, unpredictable god.)
oh god, way to make me ache with want for this.
we see you itching; don’t think we don’t see you leaning;
don’t forget how many eyes we have, child, and you forget that we were and are and always will be and that we are the most pervasive of perversions the most delectable of syllables turning your scripture to dust; the most toothsome fishthings drinking down your world and all you little scrumptious morsels tiny dumplings sticking in our gumses
don’t forget that we could eat you with our spines
don’t forget that we are legion we are hellworms we are the great abyssal flood that now at last we have fingers and toeses and a thousand million backbones all of us cosied up tight in this squirming suit of flesh and screams that our mouths are myriad and wide as scylla hollow as charybdis that we spat the whole dark night out from beneath our tongues
don’t forget now
we’re the snakings in your gardens we’re the tang of metal in your throat we’re under your feet and behind your eyeballs and scratch-titched beneath your fingernailses we’re the water you drink the fog you breathe the rain you crave and dearie-us-goodness we’re absolutely positively everywhere;
and this old angel-thing’s crookedy elbows and rackety knees will bear us far and near;
and we’ll be a-rapping down your windows and a-tapping down your doors, snickersnack below your ear;
we’re the point behind your head and don’t look now child let us lick that prayer off your lips and savour you a while and throw your boneses to the breezes
sickersuck the marrow
you can most certainly run but don’t think for a minute we don’t see you fearing;
children oh children
we invented the thing called fear
Just like the plot of Johnny English!
Some of the lesser Leviathan have complained about it in earlier episodes, like “Slash Fiction.”
I was thinking, what if Dick, being the boss Leviathan, had more access to Castiel’s thoughts than any of the others?
What if Dean has a confrontation with Dick, and asks him, “Why are you spending so much effort on two measly hunters?”
And Dick answers, “Because, Dean, you’re the most important man in the universe. Your angel friend never told you?”
And that’s how Dean finds out just how much he meant to Cas.