There's a chance for another character to join the studio, it would be after a quest to join, but this plan is for just 1 other character. No other characters will be added to this function.
There is already a solo route for Samantha, you lose most of her content but there is. As for Haniya, no, there will never be a steal route for her, you either accept Gabriel's share, or you don't. As for Tristan, those can be considered background npc, Tristan has no relation to them, but maybe if there would be time, it would be possible.
Alice is kind of bugged for the invitation but is getting fixed for the new version.
The route for betraying Zhen already exists but is very unlikely for Nyx to join for DnD.
There won't be a gallery, if one is implemented it will be only for non-repeatable scenes.
The Kimuras are not totally finished, and you might see more of them.
Just don't tell Kirby about being with Robert, or don't ask Robert about it. Other than that, there won't be a "steal" route for them.