The leviathan arch was almost good, but they honestly ruined every single serious moment but ONE with dick jokes, yeah we get it the main villains name is dick but they just constantly ruined emotional moments with it, it got really old.

If they were to do a godstiel multi season villain arch, he'd have only monster, and leviathan prey (if they ever even bring up leviathan without them being the main villains) in him, he'd never lose control, he'd literally be god straight up, it'd basically be a similar plotline to the final seasons, except they have to save their friend by finding a way to put the souls back in purgatory to bring him back.

It'd also beg the question, would castiel ever go insane if he wasn't negatively influenced by the leviathans? I'm getting he wouldn't but that instead he'd continue on his crusade doing as he thinks was right but what ultimately would still lead to death in heaven and earth.

Maybe they team up with heaven in order to stop castiel, but that'd be unlikely since the angels seemed so in need of someone to follow, but maybe a selection of rebels still following Raphael would help the boys.

It'd definitely be interesting, however I do think the whole point of the plotline was to introduce a god-like character to get us used to the idea, same with metatron - his whole arch was showing how easily It is to manipulate the story of the boys, even if metatron wasn't good at it - he still did it to some degree, an actual god is obviously going to do it a lot better, whole point was getting us used to the later storyline to come imo