Regarding Metroid fighters, the only one that I could see getting cut is Zero Suit Samus. Samus is guaranteed given that she’s Metroid’s lead and is one of the original eight (I do think that the developers hold value in the original eight specifically). Given his long journey to join SSB, it would probably upset Ridley supporters if he misses the cut next game. Dark Samus is a case where I think most echoes will probably tag along with their parent fighter.
Zero Suit Samus is kind of a weird situation where she was initially added to fill a Metroid fighter quota as Ridley wasn’t considered feasible at the time, and Dark Samus was pretty new at the time of Brawl’s planning. Despite the nature of her inclusion, I think Samus is one of those instances where there is value to having multiple versions of the same character given the Easter Egg at the end of the original Metroid. Also, I think the developers value her as a character given that she also appears in promotional material like Ryu, Corrin, and Min Min’s reveal art. If I had to rate her chances, I’m currently leaning towards her being more likely to return than being cut. Personally, I‘d want her to return as she’s one of my most used fighters.
If we include Sheik in the conversation, I’m currently leaning more favorably towards Zero Suit Samus than Sheik at the moment in spite of Sheik‘s extra appearance in Melee (I hope both return as I use them often). My logic for Sheik being less likely deals with how Sheik has been approached as a fighter from a development/promotional standpoint. In Melee, Sheik and Zelda were at an equal footing at the time given that they were an all or nothing situation, and Sheik was the more conceivable character in terms of moveset while Zelda had the recurrence and larger legacy. I assume Sheik was revealed first in Melee for being a more conceivable fighter. After Melee, it feels that Zelda was at the forefront compared to Sheik in terms of marketing in Brawl where there was roughly a six/seven month gap between their confirmations, and Zelda being the bigger focus even after Sheik’s confirmation. After Brawl, it feels like Sheik hasn’t been used much in promoting the game, making me feel like the developers are less attached to her compared to other longterm veterans. In spite of being a second Zelda with what I assume is less attachment from the developers compared to other longterm fighters, cutting a longterm veteran is probably something not taken lightly like how there was considerable weight going into Waluigi’s Mario Kart 7 or Ice Climbers’ SSBFor absence, and Sheik was one of Link’s main allies in one the most prominent Legend of Zelda games.