Like all of my plots, it's entirely up to negotiation exactly how the fated person changes, though I'm not in the frame of mind right now to turn it into a cheating plot. I had imagined it to be more of the supernatural variety, where their vitality changes, be it either making them feral and insatiable, or more and more listless. Less and less "themselves", but depending on the partner, I'm happy for it to go either way.
Ultimately, I'd need a master storyteller to build this with me. Someone who specializes in plotting and worldbuilding, even though I can't exactly see it going super long term.