I initially thought Football, but after some more thought, MMA. I've lost interest in both over the years, but I don't see what could improve in the NFL over the next 10 years that would pull me back in.
With MMA, even though I feel like the overall star power isn't what it used to be, it'll be interesting to see how the careers of guys like Khabib, Isreal Adeysanya, etc. play out.