Interesting idea. In a sense this is also kind of "decentralized" in terms of hosting because you are just using the APIs, and the service itself will just be provided via a client managing the credentials. But I don't quite understand how this will be so different from Matrix has been doing. Basically, the application is just a centralized hub with bridge to different communities. Wouldn't this just be a "yet another standard" situation?
What makes this approach really stand out is that, even if there are 5 different aggregators, it doesn't matter, because they're all aggregating the same data. And even if someone decides to use one of the existing platforms (e.g. discourse), posts/comments from this aggregator to the platform still show up. So this is actually a way to alleviate the "yet another standard" issue (though I acknowledge it won't be as good as a single common instance, due to different formats).
I think that you go to the site hub, you should immediately see a feed of curated popular posts and a "Create Account" form, similar to https://reddit.com/r/all, along with a search bar and list of filters. And when you create an account, you choose some suggestions and get presented with various communities, also like Reddit. Most people are barely even going to try your site, if you want them to join and put real effort into contributing you need to present good content as fast as possible.
Another issue with Matrix is that a lot of content just isn't on Matrix. As well as Reddit and other existing communities. For example, Rust has a subreddit, discourse, Matrix forum, and Zulip, and probably a discord somewhere too. There should really be a single platform where I can see posts from all of this, because I'm definitely not going to be checking each one individually.
