There have been theories that Sylux might be Samus' brother, and I feel that would be pretty huge, and could set him up to be a staple for future games. I'm really interested to see his design, cause he looks absolutely ridiculous in Hunters.
Oh god, please no secret Samus sibling! That'd be the most cliché thing they could do, especially after the computer in Fusion turned out to be her dad, and the main villain in Dread also turned out to be her dad
I get creating personal connections between Samus and the characters around her, but "I was your brother all along!" just seems to me like it could come across super hackneyed. I dunno.
I think it's safe to assume Sylux will get a cool redesign. I'm most curious to see Samus herself in a brand new suit, though. It's probably my #1 hope for what we see in the trailer.
Crackpot Theory:
Sylux is actually just Samus from a future where the Federation does try to hunt her down like we all thought would happen after Fusion.
The constant referring to Sylux as a “he” is a nod to the original Metroid.
Sylux in Prime 4 will be allied with the Metroid from the ending of Federation Force. Who, like the Baby/Infant Metroid in Metroid 2, sees Sylux as a mother.
That ‘Samus from the future’ thing probably isn’t true, but I hope Sylux uses that Metroid like a partner in crime rather than just another source to clone them.
It's an interesting thought, but I'm wary about yet another Samus "variant" as a main villain after the SA-X and (especially) Dark Samus. I feel like a lot of the ideas fans frequently float (Sylux is Samus's sibling / her from the future) are a bit too fanfic / on the nose to actually manifest, but who knows. At least if one of these theories did come true, Retro would almost assuredly execute it well.
The idea of Sylux allying with the Metroid from Federation Force is neat, and something I haven't seen suggested. I always assumed that particular Metroid was more symbolic of Sylux's wider plans for the species, but if it's instead a deliberate parallel to Samus and the baby, that'd make sense. I just wouldn't expect it to remain in its larval form. I think we can safely assume we'll be seeing brand new evolutionary offshoots distinct from Aeion or Phazon Metroids, and I'm excited for that prospect.
Whatever happens, Sylux's actions will surely serve as the final nail in the coffin for Metroids as a species, inevitably leading to Samus's mission to eradicate them on SR388. After Mother Brain's efforts to weaponize them, in addition to the events of the Prime Trilogy, and finally whatever crisis Sylux is sure to bring about, we'll get the complete picture of why the Federation decided Metroids are too dangerous to exist.