A few ideas from the top of my head

• Flairs that can be allowed to filter content in a community
• Major online communites (can be subreddits, or other communities) moving to Lemmy
• Reddit removing old.reddit
• Reddit banning people using VPNs (already happening, see !reddit@lemmy.world )
• Lemmy becoming the reference source of knowledge for a certain domain

Second point is probably crucial, but I don't see any major subreddit wanting to move here. StarTrek is the exception more that the rule.