Mastodon or Fedi? I will answer about fedi.
First, it's going to go in the direction email went: we will need increasingly more anti-spam heuristics, and we'll grapple with the problem of threads not having an owner who could be doing moderation on them. We will continue to have amusing reply propagation semantics, and probably will have instances do more polling to circumvent them. People will continue to be annoyed by not having a way to subscribe to a thread other than by having something poll it, so instances will implement such polling. There's likely going to be a discussion on whether such polling is reasonable, and problems caused by traffic spikes caused by badly spread out polling.
If large instances by well-known companies will appear, fedi will be split on the question of federating with them and there'll be at least some amount of defederating-due-to-their-lack-of-defederation.