WARNING. POSSIBLE SPOILERS AHEAD. UNLESS YOU ARE FINE WITH POSSIBLE SPOILERS, DO NO READ ON AHEAD.
This is something that has came to my mind when I saw a specific image. To describe the image in simple terms, it is two different times. One where Pearl is speaking to Rose, and one where Lapis is speaking to Steven. Both Pearl and Lapis say...
"Just let me do this for you!"
That is when I thought of this theory. The image I spoke about does not belong to me. It is not my art. Let's continue on the the theory.
Lapis Lazuli is meant to be Steven's future "Pearl"?
Now, now, calm down. I can already hear you saying like "But Pearl is his pearl! Why did you put quotation marks around pearl? You are wrong." I assure you. These questions will be cleared up.
I do not mean Lapis will be the pearl to his diamond. Not at all. I mean Lapis will be his pearl similar to how Pearl was to Rose Quartz. Our lovely Crystal Gem Pearl was Rose's "pearl." No one else was as dedicated to Rose (from what we have seen) nor cared as much as Pearl did for Rose. Now that would bring in one question. Lapis in love with Steven? No, no, not like that. I mean that Lapis will care for Steven, bond with Steven, and become protective of Steven just as Pearl did for Rose. She may not fall in love with him, but she will care for him more than anyone else she has met before. Why?
Peridot forced her to be an informant.
Jasper helped Peridot and forced (not exactly) her into a fusion which they kept for too long.
She distrusted the Crystal Gems for some time.
Only Steven made her smile, laugh, and happy after such a long time. They grew a bond that Lapis cared about. Steven, similar to Rose, made Lapis fell special. While not in a romantic way, but more of a friend way. Lapis has even sacrificed her own freedom for Steven and the gems. She fused with Jasper as a way to keep Jasper from hurting anyone else.
Now how could Pearl not be Steven's "Pearl?" She cares for him. She is protective of him. So why not Pearl? Well... I think that Pearl will get into some trouble. Maybe she will be poofed or captured. And someone will have to take that Pearl spot. So why not someone who Steven has, in a way, saved? Why not someone who may have one of the closest friend relationships to Steven?
And strangle enough, Lapis and Pearl do have similarities. Short hair, a thin body shape, they have graceful dancing styles, they both have shown to be caring in a kind way and a much more adult way, and the two have done something "dramatic" for someone they care about very much.