Rambling ahead
I'm making this as a kind of response to every comment and post I have ever seen in regards to safijiiva having new forms in the future, whether it be some white fatalis equivalent or a new black form reminiscent of his old concept art.
First of all, safijiiva is thematically meant to be a 'perfect being' and as such reproduces asexually, and has no need for outside sources of energy. As you can imagine, this doesn't exactly give the impression of future change being possible- and considering his mastery over regeneration, a crimson fatalis like form gained in a fight with another monster after he is matured is unlikely.
Secondly, we are never seeing that completely black form in game. There isn't really room for speculation here. Old concept art scrapped in favour of a different design has (to my knowledge) never been used in any capacity. (EDW doesn't count because the idea for the creature itself was all that existed and was then repurposed into a monster rather than a variant of an existing one being created from it) The comment about the edges of his scales turning black was also confirmed to be just a hint to his thematic rivalry with fatalis.
I also want to mention that the xeno we fight isn't substantially different from a normal specimen. It may have been slightly stronger but there is no evidence to suggest it would have matured into a different kind of Safi (perfect means perfect) all of the cocoons in the bottom of the secluded valley are also the same size as the one we encounter in the confluence of fates and so are the sheds.
Regional variants are also off the table. Safi absorbs bio energy, and every monster has some with monsters being present absolutely everywhere we know of.
To be clear, I don't want people to stop speculation or give up hope or anything. I just want people to hope accurately.
If any future variant does exist, it'll be due to either something happening to a cocoon directly or a newborn xeno getting into a fight or similar experience that would change the way it matures. All of this is to say, it wouldn't actually be a Safijiiva variant, it would be something distinct. (Or else it would thematically clash with the concept of the monster)
Tldr: thematically thematically thematically the thematically perfect emperor of elders is thematically perfect no variants ever thematically perchance
If there's anything I'm misinformed on please tell me, I just get kind of annoyed when others are subject to misinformation and regurgitate these ideas all over the internet.