After Aurora, when the gun control push didn't materialize, I said that the next attack would be a school. Half in jest.
The next attack will be on a church or something. They need a bigger impact event.
A lot of red county church goers would shoot back. That means a blue county or city attack. But they also need a whack job with a gurdge against some specific institution and that's a bit difficult to get with a church. I would lean towards a hospital with a psych ward and look for a specialist who treats some patients and then gets another job somewhere else. Not long afterwards...
Sports event?