I think the future of "SSGs" is that they're going to end up as programs running on the server that watch data from various feeds - databases, RSS feeds, APIs - and generate output from those. Basically, a streaming processor framework. The output would likely be in a "semi-baked" format so that they can contain processing instructions that are executed at request time, or otherwise lazily.

These won't be static site generators any more, and will lose out on the "you can throw it up on Github Pages" benefit, but they'll be far more powerful, and it'll be easier to develop dynamic, data-driven websites.