So, about that SUF promo that leaked.
On Steven, anger, glowy pink avatar state, and possibly the corruption theory?
Steven seems angry both times we see the Avatar state glowy pink mode. I've seen speculation that it's some superpowered rage mode where Pink Steven takes over, and it's hard to control, but I think that idea might have a bit of a backwards relationship between the metaphor and the thing it's a metaphor for.
Also, Pink Steven isn't a separate entity lurking within his gem inactive most of the time, like some replacement theory for the now GOOOOOONE idea of Rose existing within him, with the potential to take over. Pink Steven is a part of him, and always has been.
I think it might be more thematically consistent for Steven's powers to stem from his emotions, rather than his emotions being based on his powers.
Steven has spent a lot of his life having a very shaky sense of self, and being an excessive people-pleaser. He has ignored a lot of hurt targeted at himself; for the sake of not escalating situations, or because he had a guilt issue and assumed he "deserved" to feel hurt, which would mean not "deserving" to retaliate. He's supposed to be the peacemaker, the mediator, what's really important is what other people are feeling...
Steven also doesn't deal very well with anger and conflict. Probably even less so if it's himself being angry.
In the past, Steven has very rarely gotten angry, usually felt guilty about it afterwards (see: Steven's Dream / Adventures in Light Distortion), and/or tried to quarantine his anger so that it wouldn't impact or bother other people (see: Storm in the Room).
As seen in the movie, however, Steven doesn't fall for the Diamonds' little manipulations, or blame himself for things that had nothing to do with him. He's finally starting to heal from the shaky self-worth, the guilt complex, the excessive people-pleasing.
Such a recovery, however, can be a really difficult thing to go through. For someone with the kinds of problems Steven has struggled with, trying to develop your self-worth may start feeling as if as if the lack of it was the only thing keeping you in check; what if you land too far into the other direction?
It's rather like the anxiety voice telling you similar things as all those hot takes claiming Steven is a bad person in the movie for having gotten "more selfish" (meaning, less compulsively selfless than before).
So, maybe the story could touch on the themes of handling the difficulty of having to become more "selfish" for your own sake, but having to balance that, and not knowing how to manage the kinds of emotions (such as anger) that you used to quarantine away, used to refuse to allow yourself to go through.
Which... probably won't go perfectly on the first try.
It's been speculated that Steven could have been the cause of the exposion seen in the promo (screencap below). The settings, at least, do seem to match.
Whatever happens, Steven will probably not surface as the perfect person who did everything right and always handled things the nicest possible way. Time to see if that self-love actualized in the Baxter-animated dance in CYM can be unconditional enough to accept that.
(Parallels could possibly be drawn to White Diamond, who thought se always had to make everthing perfect, and therefore, be perfect. It could be worth noting that in the promo, when the angry glowy pink mode activates, it starts out looking like a blush.)
I've been kind of uncomfortable with a lot of the fandom's "mainstream" approach to the "corrupted Steven" theory, for various reasons, such as the risk of the body horror and fantastical taking away focus from the psychological issues it's supposed to be a metaphor for; focusing on the spectacle at the expense of the nuance. Beyond that, I'm not sure I like the idea of potentially having such an important character moment happen while the character is... not all there, not entirely as himself, not with his own appearance, ability to emote or communicate. I would... probably rather have Steven be recognisably himself for that.
There's a bunch of other things that potentially bother me, but that would be whole 'nother post.
However, I am not saying the idea has no potential for something emotionally impactful -- it does. It would have to be applied right, though.
I was musing on the possibility that if the corruption theory turns out relevant (although the themes would still make sense if it didn't),
what if the corruption represents not exactly the deepest pit of mental illness Steven could possibly ever sink into when it all gets to be too much, but rather, the mental health Steven would be afraid to develop, since it would mean being more selfish -- no longer in the comfort zone of being an excessive people-pleaser afraid to take up space, cause conflict, stand up for himself, because that might mean making others feel bad?
What if the corruption represented what Steven is afraid of becoming, if he lets go of his issues?
When Rebecca Sugar was talking about SUF on the podcast, she said there's some stories she couldn't tell before CYM, not when Steven had such a shaky sense of self, with the excessive need to be what he thought everyone else wanted.
Also, I do not really think it would be the greatest idea for the final lesson of SU(F) to be that someone like Steven needs to control his anger better, and think more about how him openly expressing his feelings could affect other people. That's... that's the exact opposite problem to the one he's had, so far.
What if the final boss turned out to be that mental illness anxiety voice that internally tries to prevent recovery by trying to convince you that the mental illness was the only thing keeping you in check, so if you did recover, that would somehow mean becoming a selfish monster, taking up too much space and attention, and hurting people?