Hello. I don't normally do theories about future events, but this one was too good not to share. I think I've posted this theory on other thread, but I seemed to have misplaced it.
Anyway. I've heard theories that the new Gem is "Red Diamond" or "Pink Diamonds replacement", I'm going to take this even further. My theory is that the new Gem, that I've decided to name "Bort" (for those who don't know, bort is a term for an imperfect diamond) was actually made from the "shards" of Pink Diamond. The ones she made and swallowed to fake her shattering.
If you watch "Change Your Mind" you'll notice what look like bubbled cluster gems in Yellow Diamonds room. This means that the idea to fuse Gem shards may have started on Homeworld, and were made by Yellow.
My guess is that the original idea for combining Gem shards was to resurrect shattered Gems. After many experiments, they decided to reassemble Pink Diamonds "shards" and bring her back. Of cause since they weren't realy Pinks own shards, they formed into a completely different Gem. Eventually Bort was bubbled and the resurrection project cancelled. Of cause Homeworld for another use for Gem shards, but that's another story.
Your probebly asking "How could fake Gem shards make a new Gem?". Well, Pink does have the ability to create life and the shards were made by her useing her powers on soil, so why not?
But that's just my theory.
Edit: After seeing the trailer and doing some thinking, I've decided to add to my theory.
As we all know, the new Gem (who I'm still going to call "Bort") is very rubbery and can stretch her self like Jake the Dog. This doesn't seem to be standard shapeshifting as it's a lot more fluid and her parts don't glow.
Also, her stretchiness kind of reminds me of "Indirect Kiss" When Amethyst cracked her gem an went all noodle. If my theory about Bort being made from combined Gem shards is true, then it my explain her stretchiness. Sins her gem is not one whole piece, but rather a bunch of pieces, her form would be unstable and not solid. This may also explain her craziness, not only is she physically unstable, but mentally as well.