Some think we will get Helia, a Halfling Bard Werewolf datamined early, but sje disappeared. Minsc was also originally datamined to be an origin character, but got demoted to none origin character. I think Helia's history. Also Minthara and Halsin were not originaly entended to be Companions at all, they get promoted to none origin companions and they will have to change their stories from EA accordingly.
My guesses are Alfira whose a pretty Tiefling Bard with a cool outfit and who sings beautifully, that many have been begging to make a Companion, yet she wasn't listed along side Minsc, Jahiera, Minthara, and Halsin as none Origin Companions. I think its because she could end up the last Origin Companion.
Or perhaps because Minsc is no longer an Origin Companion someone tied to the BG video games or Comics became the last Origin character, like Viconia, or more likely because no companions are Sorcerers, Delina, who turned Minsc from stone to flesh. She'd make sense as a substitute for known character Origin character.
Alfira is definitely most-hoped-for and has featured really prominently in a lot of released art, screenshots, videos, and so on. At the very least I expect she'll travel to Baldur's Gate and serve some role there, because like, that's a lot of effort to expend on a character otherwise. She's also a Bard which, honestly, would be by far the most
useful class to have as a companion (because they have a ton of skills, a ton of actually useful but not spotlight-stealing spells, and can buff the heck out of other characters, and heal!). One of the issues with the current selection is that, sadly, they are not the most
useful bunch of companions in the world. Especially as as Halsin is a Moon Druid rather than a more support-oriented one.
However given how cryptic they're being I kind of have my doubts. She could always come in as DLC or something later.
I think Viconia is very unlikely given there's a Half-Elf Cleric and an Drow Paladin already.
Delina, with respect, is not going to happen. She's a deeply obscure comics character and would just make fans go "Who?", so trailing her as an exciting final Origin character would be asking for fan backlash - especially as she's Yet Another Bloody Elf (TM). She'll likely be in the game, but playable? Nah.
There's some speculation re: Edwin/a or some sort of variant on Edwin/a, because part of the ARG seemed to hint at it, but I'm pretty skeptical.
I think in general any human or elf or half-elf is unlikely because of the sheer number of them already. Larian certainly seem to think people will be excited by the final reveal, so I tend to suspect a more exotic species (like Dragonborn) and/or a fairly well-known BG lore character (Imoen should be too old but that means nothing, and she had a very unconfirmed death in the lore - but as a human and doubling up Rogue I think she's unlikely).
Helia seems plausible at least, though I have a feeling that they might have backed off on her.