Any of that could be done; there's some parts that are more challenging but there are certainly harder things that have been solved by open-source software. I know almost nothing about how Lemmy's innards are built though, so I couldn't hazard a guess as to how much effort any of it would take. Some of it could possibly be achieved through separate services that you could host alongside a Lemmy instance, or entirely on their own, while other parts would really work best as features within Lemmy's own codebase.