I don't know how many times I've watched it now...
I had an old thought occur to me...
well, more like a very loose theory based on what little I know of regarding Steven Universe.
This one is about Lapis Lazuli...
More specifically about why I think Lapis would never fuse again after the crap between herself and Jasper.
Now, what I'm about to say is mainly from one specific point in the series, but please understand that my current knowledge about Steven Universe is based on clips I've seen on YouTube as well as the two-hour video done by Lily Orchard.
Now, which clip am I talking about that's actually canon?
Funny enough, it's only a small bit from Alone At Sea, specifically the clip below at around 4:27.
Jasper remarks here that she's the only one who can handle Lapis' power.
Mind you, this is coming from Homeworld's ideal warrior: if memory serves me correctly, even Peridot (from a technical standpoint) says that Jasper is perfect.
It might be insignificant information... but what if it wasn't?
In Too Short to Ride, Peridot finds out that she's a stunted Gem, like Amethyst. This means that, especially from a power standpoint, she is not at the fullest potential she could have been if she wasn't.
What does any of this have to do with fusion? I say this theory is loose because this theory also hinges on a head-cannon I have about the Steven Universe's... well, universe.
Even though I don't have any 100%, without a shadow of doubt evidence to prove this yet, I believe that the reason fusions can be hard to control (Sugilite being the most infamous of them, as I've heard she's gotten really bad reviews by even the black fan-base) can be for a number of reasons: the number of Gems, how well in sync said Gems are, etc...
I also believe another factor that plays into the stability of a fusion is the power levels of each respective Gem.
You see, if there isn't a balance in power, then even if fusion is possible, it may be harder to sustain the said fusion. That explains, at least to me, why Jasper said what she had said at the time to Lapis.
Think of this like the fusion presented in Dragon Ball: Trunks had to knock down his power level a little so he could fuse with Goten.
Using this same principle, then even though this would likely piss off a lot of Lapidot fan-base, the likely-hood of a fusion between Lapis and Peridot would be all the less likely, if not impossible.
Even if they never fuse on screen not just because of the priorities of Steven Universe Future?
What if the reason they never fuse involves the power dynamic between Lapis and Peridot?
What if the result of that fusion created, at best, a hard-to-control fusion because Lapis is such a strong Gem, while Peridot (by power means) is on the opposite end of the spectrum?
This isn't just Lapis and Peridot: this would also be a good reason why it would be risky for any stunted Gem to fuse, or at least make an interesting point/episode of what could happen if two stunted Gems were to fuse - for example, Peridot and Amethyst.
Now even if a balance of power wasn't an issue, let's not forget the crap that Rebecca Sugar pulled between Lapis and Jasper. What Jasper did to Lapis, and vise-versa, will forever change how Lapis looks at fusion and essentially the relationship she has with others.
During my lifetime, I've come to understand that there are situations where you have to forgive people: not because they suddenly decided to not be scumbags anymore, but rather for the sake of your own emotional health. Long story short, you forgive them for you, not for them.
(As a side note, if forgiving people for you was the central theme of Steven Universe and not redeeming the worst of people, then I would be a little more lenient with this show.)
If they had at least done this with Lapis (forgiving Jasper for her own sake and not because Jasper is just the one of many to get an unnecessary redemption arcs), then maybe I would understand why she is the Gem she is now.
However, they don't: do you honestly expect me to believe that Lapis would be careless about a fusion with just anyone? Of course not.
In fact, even if she did forgive Jasper for the stuff she's done (and forgive herself for the same things she did to her), that doesn't change the fact that she wouldn't trust Jasper as far as she can throw her, and from what we've seen in Alone At Sea, Lapis can throw Jasper pretty far.
Just because you forgive someone doesn't mean you trust them.
Add on the potential that Lapis could also have issues trusting herself in fusion too, and that's three reasons why Lapis may not ever fuse with anyone ever again.
My point is that if power-dynamic isn't an issue, personal demons would likely prevent this Gem from fusing in addition to the fact that we've yet to see her work through those demons: she just went from being a depressed powerhouse to being a remotely okay Gem. If she ever does fuse again, then she better get her sword out to fight those demons because if she fuses with even her closest friends, I demand an explanation.
Don't write on what you can't deliver.
