I think that one of the most important steps we are going to take towards Castiel being something other than comic relief is going to be seeing him in different clothing.
As much as I like the ill-fitting overcoat and suit - it’s comforting, the silhouette immediately recognizable - it has become such a defining characteristic that some are unable to picture him in other garments. It’s the suit, overcoat, and blue tie. That’s Castiel.
But let me lead you on a mental adventure. I’d like you to picture a cartoon character. Any cartoon character.
Did their clothing ever change for anything more than a one-off gag? Did they have a wardrobe of different outfits? Or was it the same clothing, episode after episode, because the clothing was a part of the character?
Sure, the Winchesters are more often than not swathed in some concoction of plaid and canvas and denim, but rarely do they wear the exact same clothes even throughout the span of an episode. They change clothes to show time passing, to show their characters advancing, to show them becoming other people entirely when the occasion calls for it.
But Castiel has always just been Castiel. The only variation we have gotten was Castiel in the white scrubs of the mental hospital, which was also a constant for several episodes - done, I think, to differentiate this broken Castiel from the one that we knew so well. Which is my point - the clothing was used to show that Castiel was different.
(I refer to Castiel exclusively here - his time as Emmanuel of course had a wardrobe change because Emmanuel was a different character, and note how as soon as he was Castiel again he donned the overcoat; Jimmy is also another matter. Both Jimmy and Emmanuel were discrete characters, decidedly not Castiel.)
Castiel has always been somewhat frozen in time and plot development. He has always furthered the Winchesters’ plots, never truly his own. Even in the rare story arc focused on him, the context has always been in the frame of the Winchesters’ current goals.
He’s been comic relief. He’s been a cartoon character, a constant we as an audience did not need to see grow or develop because the story was never about him. He would swoop in to save the day - a superhero uniform, another immutable costume - or get the Winchesters through some convoluted plot device, but as a character, he was a caricature.
I think that this coming season, that may change. Unless he lines his closet with overcoats and black suits, we can probably expect to see his clothing closely reflecting the Winchesters’. Stripped of the constant, he will change. Grow. Develop.
He’ll cease being Castiel and begin being Cas. And while heart-wrenching in a way, it’s also a great thing to look forward to.