I've been to a few. They still have really impressive feats of human skill. The one that had animals in it had the animals very rarely and seemed to be taking better care of them (I think some horses trotted in a coordinated pattern and an elephant came holding/wearing a banner, then left).

I'd be interested in seeing how they handle holograms. Circuses seem to try to push visuals and entertainment. They might come up with some really creative new ways to show off.