I'm a big CRPG fan, and so crafting party combinations is among my very favourite things to do in a video-game, but I think it'd clash with many of the elements of what makes the Witcher series so special. It's very much a personal journey for Geralt, and the people he meets along the way help shape it, but they don't necessarily define it. There's something to Geralt's adventures that's always seemed morbid to me as if it was borderline a suicide attempt, there's just a desperation in what he attempts to accomplish, as well as the odds he goes up against.
It's actually a subject of the books, where-in he constantly seeks to set out on his own, even when he has people who care about him and wish to help him. It's basically because his mindset hasn't ever been practical. The challenges he goes up against aren't the kind a single person can realistically defeat. He realizes that, but he's emotional in his own way. He just can't sit still, and so, his quest has at every point been more about a trial of fire, and a pushishment for himself than anything else. "The fire hardens you, but it'll also leave you crippled" is a quote you'll see a couple of times in the novels. In the books, he actually does meet some other people who force their way into his life and form this kind of adventuring company you're suggesting.
A point that's already been made in this thread is how anyone could possibly keep up with Geralt, him being a mutant et all. (Spoilers for the books)
They can't. His friends all die, one by one. And he, naturally, blames himself for giving in and accepting their help, despite having known it'd end that way. So there's no way he'd ever put himself into that situation again. The short story "The Sword of Destiny", which they've been mentioning in every single trailer of the game, is actually about this grief and guilt of his. "The Sword of Destiny has two edges. You're one, and the other is death."
And that's why The Witcher series won't ever be about a group, or adventuring party.
EDIT: I just realized what a fucking Witcher nerd I am.
I never got a chance to read "Sword of Destiny", but rather read a summary of it. And, based on that summary, it seems the short story I got was not the same story you just told me.
But I understand what companions you were talking about, and they were from the novels. They included Milva the huntress; Cahir the knight; Regis the vampire; Angouleme, who looks just like Ciri; and Geralt's long-time sidekick, Dandelion. All four of them would end up dead helping Geralt rescue Yennefer and Ciri, while Dandelion would outright leave the group altogether to be with a duchess.
Still would have liked Geralt to carry around AI-controlled party-members, though; after all, he had two sorceresses; an adoptive daughter whom he trained in sword-fighting; a bard; a dwarven warrior; and either Vernon Roche or Iorveth, depending on who he sided with in Witcher 2. Plus, there was another group of superhuman warriors similar to Witchers called the Grey Wardens from the Dragon Age series, and the main-protagonist, who was a Grey Warden him/herself, carried around party-members who were not Grey Wardens themselves like the protagonist and Alistair. Same for the Spectres from the Mass Effect trilogy; Shepard was declared the first human Spectre for her fighting abilities and leadership, and yet she carried two human Alliance marines and four aliens who were not Spectres themselves, yet fought with as much brutal efficiency as Shepard herself.