One nice (yet sometimes annoying) thing about Lemmy is that you can have multiple communities of the same thing.
What I think will happen is that a few instances of Lemmy will become the big ones and their communities for memes, news and politics will dominate.
I can even see something happening to remove duplicates. Perhaps lemmy.world and lemmy.ml agree that /politics is on lemmy.world and /news is on lemmy.ml
App developers will make those default communities easy to find. Kind of like how reddit used to have 50 or so default sub reddits.
Less popular instances will have shadow communities that will be more difficult to find, but where there will be a more hardcore group of contributors and members.