I'd suggest that Imgur will be a nearly complete replacement option for Reddit (no coincidence, it's intentional). At the rate Imgur is increasing in size, and the rate at which they're moving toward social, this is seemingly just a matter of time now.
I have a sneaking suspicion that there is a big overlap of the early adopter core user bases from kuro4hin, slashdot, reddit, and HN - i.e. a lot of the same people moved from one to the other and created the editorial direction (so to speak). By that measure HN is 'the next Reddit'.
Unless HN starts supporting sub“HN”s (or starts allowing porn, jokes and My Little Pony-related posts on the frontpage) I don't think it's a viable replacement for Reddit.
It's already got a lot more political stories (though necessarily post-Snowden); HN feels pretty much like early reddit now. Though I do think Reddit morphed its community precisely to avoid just being another in this list (Digg exodus helped that too of course).
But not the people. Empty forums are boring. They have the mindshare. There were alternatives to Twitter (some of them open source) but no one has fled to them no matter how much they cripple the APIs or how many ads they put.
Remember MySpace? They were the new hotness until they imploded. Reddit could easily turn into yet another ad-festooned wasteland, and many people are tired of the "piss off your users as much as you can get away with" business model.