I think that would be useful! I also think that anyone putting that much work in should look into hosting their own server, because they've already done the hardest part of hosting a server in the fediverse. A big part of the issue is that a lot of ActivityPub apps don't really have granular enough customization baked in to support that sort of thing just yet; you can get some apps that do that on the user side, but anything on the server/community side is usually just "block all" or nothing. The admin of my Mastodon instance is always complaining that he can't just hide certain instances from the "all" tab without blocking then entirely, and he just wants to hide them so they don't overwhelm the server, not block them from showing up for people who choose to interact with them.

Honestly, I've seen the Fediblock thing on Mastodon, and it's…pretty terrible. A whole lot of minority groups get targeted disproportionately by that stuff, especially by misinformation about their instances. The answer is really to leave instances if you disagree with moderation policies and the admins won't listen, and to join instances that are philosophically aligned with you, because unlike in a centralized/capitalist model, this actually works at cultivating a community that you can engage with in a healthy manner. If not, and you go with something like Fediblock/the one big blocklist site, you're just gonna end up with most instances that serve 2SLGBTQIA+ people getting blocked or having more harmful misinformation spread about them. Hell, if a lot of Lemmy had its way, anything but being capitalist and pro-USA would be banned.

But also, a lot of clients can subscribe to feeds already. ActivityPub is pretty great at cross compatibility with Mastodon and the like. You just subscribe to someone who uses a microblogging platform based on ActivityPub and it'll show up in your feed.

All my notes for tabletop RPG stuff, mainly! I run a few Pathfinder campaigns.

Trillium is my personal choice for self-hosted notes. I haven't really had issues with using it on mobile, but I also just tend to put the stuff I think of when I'm out and about into a single note that I periodically go through and reorganize. It's been good to me so far, and it has all of the features I really need. If I need something fancier (or public-facing), I toss it in BookStack instead. Then again, I don't use either of them for business (mostly for tabletop RPG stuff and instructions to friends/family about using the other stuff I self-host), so if that's your application, I have no clue how it holds up.