Jousting. Because it's awesome.
Absolutely this. Awesomest thing ever. Impractical as all heck though, but man.
I'd also go for horse archery as someone mentioned. Again, no real pool of talent to draw from but man it would be awesome. And tough to be consistent in.
Also I may add, that Rugby, has so little competition its not even funny. Sevens is pretty fun to watch, and will be interesting to see in 2016, but Football is just as fun to watch and would be great to see supported in its spread.
That's just disingenuous when talking about American Football in the same breath. Rugby has six solid contenders (Aus, SAF, NZ, Fra, UK, Wales) and a dozen competitive national teams. American Football has ONE. As for the bolded part, mileage may vary, and I personally find it unwatchable.
Rugby allows for smaller players, less specialists, and way way less infrastructure investment. It requires way less time to play. It's suitable to world audiences who like a game with constant pace. It's basically more suited to be a world sport by a several light-years, but it's still not very popular even so. American Football is just not even a contender if Rugby isn't.