One of the most requested features on Mastodon's github is for backfilling old posts on remote profiles when they are followed, so that followed profiles are never empty.
This would make federation much much more intuitive.
If you're comfortable using Github, and if you agree this feature would be a good idea, could you give it a thumbs up so they know there is demand for it?
➡️ https://github.com/mastodon/mastodon/issues/34
Thank you!