Anyone well known who wants to speak out about what's been happening on reddit? Louis Rossmann? Apollo dev? John Oliver (one can dream)... or maybe former Reddit mods who were kicked out?
Anyone who has a story and who understands they'd have a massive impact by giving an exclusive AMA on a Lemmy or Kbin instance.
This could be announced a few days in advance to make sure all remote instances follow the AMA community.
Of course, for people like mods and developers, you could quite easily reach out directly. The Apollo dev is on Mastodon, the Reddit mods probably responds to reddit DMs.
As for actual celebrities, you'd have to get their attention somehow. They have publicists whose job it is is to filter out the people who want to talk to them.