Well, Matrix is all of that already, it uses its own protocol also called matrix, but its properly federated(unlike bluesky) and its open source. It also has a big userbase. The Matrix protocol is seen as a honorary member of the Fediverse. I think using activitypub may be a bad fit for real time messaging, so them using their own protocol makes sense. There are also XMPP server and clients, who do similar things to matrix, also federated and open source, however they are more like Whatsapp/Telegram etc.
Well, Matrix is all of that already, it uses its own protocol also called matrix, but its properly federated(unlike bluesky) and its open source. It also has a big userbase. The Matrix protocol is seen as a honorary member of the Fediverse. I think using activitypub may be a bad fit for real time messaging, so them using their own protocol makes sense. There are also XMPP server and clients, who do similar things to matrix, also federated and open source, however they are more like Whatsapp/Telegram etc.