Interesting idea. It sounds like a federated and open source version of the “everything” apps people like Elon Musk want to create with X. The only difference would be that this is for social media.

I think there are a couple of apps that already exist that let you sign into both Mastodon and Pixelfed.

I know that Fediverse apps can communicate with each other (like you can look at and reply to Lemmy posts from a Mastodon account), but it would be even cooler and more useful if you could use your Mastodon credentials to sign into a server that ran Lemmy for example in order to get the Lemmy UI that is better suited for large, organized comment sections that are organized with lines and can be minimized instead of Mastodon style where you don’t have any of that organization. I think that an SSO-type feature like that would also negate the need for having multiple Fediverse accounts, account for Mastodon, Lemmy, Pixelfed, and PeerTube, since in that scenario you wouldn’t have to create a Mastodon account if you were already on Pixelfed just to get the Mastodon UI and style of comments.