This is something I've discussed multiple times before, but it's a topic I find most fascinating with understanding and observing fandom and its evolution over the years.
When it comes to where fandom will congregate in the future, that's kind of impossible to answer because I don't think there is a true answer for this. Over the years, fandom as a community has grown and expanded beyond just particular sites. We've had places that seem to be where most fandom would go to, before it was LJ and then it became Tumblr, but even beyond those many fannish people would go to different places to find their fandom fix on various websites. And with the Internet growing and expanding and different places where people could go to, fandom ended up becoming more fragmented. And when it comes to popularity of one platform over another, that all happens accidentally. LJ was never made to have been the main hub for fandom, neither was Tumblr for that matter. Unless there is a site similar to that of AO3 where it's specifically made by fandom for fandom, people are going to migrate to whichever site is the easiest to navigate.
As for what I'm hoping for the future of fandom in the next several years, I'm hoping that we can come back to the sense of actually having discussions rather than "discourse" (as that has become more of a "everyone yelling over who is more morally right" kind of purity wankfest). People really need to step back from thinking that everything is ~super serious~ and start understanding that you can still like something while thinking critically about it. Yeah, fandom wank will inevitably always exist no matter what, but I really want to bring back the "fandom and let fandom" aspect. People are taking things far too seriously and need to understand that fandom is not activism, people can like and enjoy things, and vice versa, without needing to justify their reasons.
Essentially, I just want fandom to come back to common sense and realize that purity culture is immature and toxic, and we desperately need to get away from that dangerous mentality.