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'.
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.
