Operating system wise I'd say WebOS should have been the next big thing if HP hadn't bought Palm and killed it, I doubt the fact that they're making it open source will have any effect now either. Shame really as it truly is/was a great OS. I'm not saying that as a "fanboi" either.
Personally I don't think we're going to see any "next big thing" that will steal the iPhone's crown for some time, 3-5 years at least. Although we may see some interesting things in that time, I guess the new camera+phone (instead of phone+camera) trend would fall in that bracket. Laser keyboards maybe? Phones with projectors?