I don't think that it would make any sense narratively to do something as cruel as take away the ability for her to touch anyone. There's no way she'd be unable to do that with a new heart.
However. That being said, one big trade-off that COULD work and be narratively alright is that she would be unable to rage and use soul coins for the remainder of the game. Raging and being a powerhouse has been her virtue for the decade that she's killed and battled for Zariel. She is excited to have been juiced up with the first upgrade and even moreso when we get soul coins.
Obviously with the engine gone, coins are of no use. So she can't be buffed for the final battles you wanna play out depending on where in Act 3 you are. As to not being able to rage, she could be forced to calm tf down as to not overdonit for the newly attached heart that yes, is magically healed in and all, but given how much power and force she's been able to unleash, that heart can never achieve.
That's the whole reason she got mutilated. To be more. More force. More power. More fire. Now if we take that away, left is a Karlach that would've grown up and been if she wasn't sold by Gortash. Does that make her less of a fearsome warrior? No. Not at all. That's always gunna be a part of her. But it's a bit like undergoing a big surgery that changes your life and you can't do 100% of what you used to before, as your body may not be able to handle that.
I think her being less powerful despite her level, unable to rage which is a big thing for barbarians and her in particular, and having to calm down from all the fighting adventures she thought she'd go on for the sake of calming down and settling a bit earlier in life (tavern, stories, brawling) would be a good deal. That way she pays a price like Astarion and Shadowheart, they can live full lives, but there's one thing that always is less than what it was. A price paid. But not a cruel one.
