It shouldn’t be a s dangerous idea. It doesn’t matter what email service you’re signed for - you can still communicate with any one in the world. It should be the same for Lemmy. Lemmy UI should be improved to better search and present communities from all over the Fediverse. And join-lemmy should have a random link to a single registration page on a random instance. That will not only make the process better for end users, but will also manage the load between instances, making the whole network more resilient.