If you're looking primarily for discussion, then I'm not sure I have anything to recommend as I don't see much of it these days either. However for recs and general fannishness I can recommend two things.
First, if you are willing to consider Tumblr, I have found that a number of active rec communities exist there as well as ones that promote news and gifsets. I can't speak to Harry Potter specifically since I was never in it, but when I activated my account there I was able to do searches for particular hashtags like pairings, and then looked to see how active those communities/accounts were. For those that had activity every 1-3 three days, it was really helpful in keeping up with news, new photoshoots and appearances, as well as new fanwork content. I'm subscribed to about six of them and I find that's enough to keep me busy each day (Tumblr is a lot more spammy than LJ/DW).
In terms of finding fanworks, especially if you're interested in smaller fandoms or rarepairs, subscribing to a feed from AO3 is really useful. Merlin has dropped off a lot in activity since the series ended some years ago, but I still get at least a dozen new items coming across the feed every day. For fandoms that are still really active (as I suspect HP is) it would be best if you subscribe to feeds for the characters or pairings you really want to read, otherwise it would be overwhelming.
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First, if you are willing to consider Tumblr, I have found that a number of active rec communities exist there as well as ones that promote news and gifsets. I can't speak to Harry Potter specifically since I was never in it, but when I activated my account there I was able to do searches for particular hashtags like pairings, and then looked to see how active those communities/accounts were. For those that had activity every 1-3 three days, it was really helpful in keeping up with news, new photoshoots and appearances, as well as new fanwork content. I'm subscribed to about six of them and I find that's enough to keep me busy each day (Tumblr is a lot more spammy than LJ/DW).
In terms of finding fanworks, especially if you're interested in smaller fandoms or rarepairs, subscribing to a feed from AO3 is really useful. Merlin has dropped off a lot in activity since the series ended some years ago, but I still get at least a dozen new items coming across the feed every day. For fandoms that are still really active (as I suspect HP is) it would be best if you subscribe to feeds for the characters or pairings you really want to read, otherwise it would be overwhelming.