It depends what you're looking for. To replace the link aggregation features, I've gone back to using RSS over the past year and I'm pleased with it. I get the same news that is spread across the various subreddits, and often times news sites will report on stuff directly from reddit.

For the actually discussions or more niche content, there isn't much. There are some Reddit-specific alternatives popping up, but unless Reddit itself really dies I doubt they'll get much traction. Your other options are the other social media platforms, which have their own issues and aren't quite the same.