Torhelm
Jean Grey would make most sense. She kind forms a duality with her pure fire nature versus his pure ice nature which is almost an inversion of their yin and yang. Also she is the only one who innately would have the potential of becoming an immortal goddess on her own. So as for the most sensible decision I would say her. If you instead want to have a less sensible decision where MC has to put in a decent bit of effort to make things work I say go for Natasha Romanoff. Who doesn't like an ultra hot broken girl. We all believe we can fix them.
Having romance in the story is quite wired as we all know it an old man going after young girls and he has a goal bot o well out of this list Jean Grey is the obvious pick as not only is she a powerful telepath she is also the host of the phoenix force and has the highest potential the rest don't really bring anything to the table. my advice is to just use an OC or stella instade