To be honest, the answer is probably "whoever happens to be in second place when Magnus falls off". Magnus has openly stated that he finds classical chess boring, stopped playing in the world championship due to the immense amount of prep required, and has started pushing for other formats (more blitz, Fischer random). I could see him just prepping less and losing his 20 point advantage by the end of the year.
The 2024 world champion Gukesh Dommaraju made a pretty huge leap this year, is only 18 years old (youngest world champion of all time), has a really unique playing style, and is likely to continue improving next year, so it's possible he'll run away with it, but I think it's unlikely. At the top levels right now, a lot of the ELO distinction comes down to finding ways to consistently dominate players that are slightly worse than you, and Gukesh doesn't seem to be optimizing for that.
