Google will likely be expanding implementation support for rich snippets through JSON-LD. John Mueller confirmed this on Twitter after a webmaster said he saw websites using JSON-LD to power their product reviews rich snippets.
Using JSON-LD is not in Google's help documents for rich snippets but John Mueller said they are looking to expand it in the near future.
Here is John's tweet:
@Hermikus We're working on expanding JSON-LD support, so I'd hang tight.— John Mueller (@JohnMu) January 26, 2016
It seems like it is coming pretty soon for John to say "hang tight."