For forums, logging into the forum with your fediverse account would actually be ideal since you can use a forum-style interface to navigate the topics, which would be easier than trying to navigate the same topics on Mastodon. Same thing with PeerTube. You may want to watch videos on PeerTube and comment right on their website without creating a PeerTube account. With federated single sign on, you can post with your existing fediverse account.

We have that on Hubzilla and it is called OpenWebAuth. We can log into other instances and comment directly on their instance as ourselves after logging in.