• People who came here expecting a monolithic Reddit 2.0 will either leave for the hot new thing or centralize themselves on a few major mega-instances.
• Smaller instances will be their own communities (in the general sense) and those that are similar enough will form "neighborhoods" (not in a defederation sense but in an unofficial "hey we talk between each other quite a lot" kinda ad-hoc sense)
• There will be Lemmy forks (or several completely new software) that's explicitly aimed at the above mentioned smaller instances
  • Think of Glitch or Hometown over on Masto, or Akkoma
• In a defederation sense, there will be a large and complex venn diagram of instance connections as instance admins take stances between reddit-style free-for-all and safer spaces (and yes, the occasional completely freeze-peach instances that'll get defederated from both)
  • Quite a few smaller instances will defederate or otherwise limit communication (assuming the options will exist) from above mega-instances as they'll be extremely lax on moderation to not lose the Reddit free-for-all audience.

TLDR: Just look at the microblogging side of the fedi.