meh
I can see metal detectors at the main doors and having security personnel around and all that, but let's not turn this into a police state.
When you think about it, a sporting event is a bit tricky if a nutjob wants to cause problems. Traditionally the best casualty causers was fire, but tha'ts mostly gone with buildings made of concrete. The rambo gunman thing wouldn't work if you're within gun-grabbing distance of a dozen people, and getting to someplace like the rafters or a private box is near impossible. Only thing left would be a bomb, but you have the same problem of having to plant it while being around a bunch of people who could potentially stop you.
Doesn't feel like something anyone could effectively target as a lone wolf or even small group. Only thing I could see is flying a plane into a building, and the way things are these days, that is just about impossible as well.