I would:
Allow users to see their followers
Allow mods to see stream followers
Allow banning a user from a stream
In addition to NSFW warnings, I would also let people put trigger warnings on their images
I would let people make custom icons (choosing the color of the icon, choosing the symbol, etc.)
I would allow users to DM or PM (direct message or private message) each other
I would allow users to "friend" each other
I would allow users to post things other than memes in streams besides the default streams (polls, statements, rating, etc.)
I would allow users to put a bio in their profile in addition to the tagline
I would make themes other than Dark/Light mode. For example, Tumblr has a theme in between orange and light brown, a neon green and black theme, a yellow theme, etc.
I would also add an imgflip emoji keyboard, with icons like meme man, upvote and downvote buttons, doge, etc
That's all I can think of at the moment :)