To jump in on the confusion, I'm American and have seen it used both ways, completely dependent on context. When I read the original comment I understood it as 'next thing that we all think is best' rather than 'second best.'

But after reading the comments and thinking on it, I realize it's probably used more often in the context of 'second best.'

Another American here -- never heard "next best" meaning anything other than second-best to what you're currently looking at.

"Next big thing" is commonly used and unambiguously means what's intended here.