It is a terrible idea to make a scenario where Geralt's choice between Yen/Triss puts Yen/Triss in danger. We are talking about two independent and powerful sorceresses that could turn Geralt into dust. Mages have annihilated the Witchers in Kaer Morhen before. It also would be so cliche.
I've never subscribed to the idea of "power levels" a.k.a. X being objectively stronger than Y and thus never in danger of being bettered by Y. There are so many different factors at play when two (or more) people fight each other: form of the day, terrain of the battle, prior injuries, preparations, ruses, weather or lighting conditions etc. pp.
The best, most powerful sorcerer can be brought down by treachery. The best fighter killed by a knife in the back or an arrow from afar, the best swordsman surprised by a pitchfork to the gut .
Some of most crucial encounters in Sapkowski's books (especially Lade of the Lake) end with the weaker party walking away victorious because they had one crucial advantage over their otherwise better-equipped and overall superior adversary.
Also, I think you misunderstood the nature of the scenario I proposed. I wasn't suggesting that Geralt would kill one of the ladies or something akin to that. I was talking about unintentional repercussions of earlier decisions that might jeopardize certain characters if not lead to their demise.
Let's say - for the sake of the argument - that Geralt and Yennefer at one point during the game are at odds with each other. Yennefer is commited to supporting the North's war efforts against Nilfgaard (because reasons) and tries to convince Geralt to take up arms and join the fight. If you agree with her line of reasoning (whatever that might be) you are on good footing with her, making a reconciliation of your prior relationship more likely (though not necessary). However, if you are in opposition to that idea (because your priority is finding Ciri) you have an argument and part ways. At one point Yennefer, due to her involvement with the Northerners, gets ambushed and is injured by Nilfgaardian special forces. However, this wouldn't have happened if you were at her site. You would have dissuaded her (admittedly an unrealistic notion
) from taking the trip on which she is attacked or at least accompanied her on it. Later, during the climax of the game (where, naturally, your paths cross again) you will have to fight the big baddy together. Yennefer, not completely healed, can't react quickly enough to evade/repel an attack and is critically hurt and succumbs to her wounds.
Another example: Geralt fights an important adversary midway-through the game, gets the upper hand and is ready to deal the final blow. Now you have the choice to let your opponent live or simply kill him. Since you sympathize with his cause and there's no real animosity between you - the duel being one of those unavoidable situations that sometimes simply arise between people with different agendas/wordviews - you mercifully decide to grant him his life. Then, towards the end of the story, that person, still fighting for his side, appears again during a crucial battle. Triss is also there because of another decision you made. She could have been occupied with saving Dandelion who has gotten himself in all sorts of trouble (naturally) but you wanted her at your site during those pivotal moments and told her to stay in your vicinity (you instead send Zoltan to rescue the bard). In the midst of the fight Triss incinerates the guy's comrades with a spell. He gets understandably pissed and approaches her from behind. You see this, try to come to her help, but because of an injury to your knee that you sustained earlier and haven't let heal properly (although Zoltan told you to seek treatment, two seperate times) you stumble - and have to watch as Triss is fatally stabbed.
Another scenario: You break up with Triss because of Yennefer. Triss is heartbroken but remains with you. During the end fight, both Yennefer and Ciri are incapacitated due to some strong magic that is employed by the bad guy. They are about to be killed and you only have time to save one of them (which could have been avoided if you had taken certain precautions, but you didn't since you were occupied otherwise). You rush to intervene and stop whatever is happening to Ciri thus basically sacrificing Yennefer. Triss, despite her hurt feelings, steps in and takes the blow directed at Yennefer. She dies.
This is obviously a pure thought experiment meant to illustrate the kind of gameplay mechanic I was thinking of. An interconnected chain of events that is influenced by - over the course of the game - a number of your decisions which might seem irrelevant to the larger picture at time but can potentially have far-reaching consequences. Please not that the situations decribed above most likely don't hold up to scrutiny and shouldn't be taken seriously, neither story nor character-wise. It's simply the first best thing I could come up with (at so late an hour).
Edit: Also, these are three variations of one possible scenario (a potential love interest dies). Of course this cause and effect/action-reaction principle could apply to a much smaller, less deadly range of situations. Who stays on or ascends a throne, what happens to the Lodge, the fate of the Scoia'tael and so on. All these things could be affected - to a reasonable degree - by several choices that have an cumulative impact. Whether Geralt ends up with Triss or Yennefer (or none of them) could be dependent on a number of minor decisions that have an influence on their behaviour towards him as well as on parts of the ongoing plot.