Need thousands of new major articles
It would be good to have lists of needed articles, which are guaranteed to satisfy notability requirements. Then somehow, suggest to new users who are trying to create new articles for the first time, that they start with a topic which is guaranteed to be notable and have good references available to use. When new users pick their own topics to write about, they often overlook the "needed" articles and pick something of marginal notability which requires far more editing skill to make deletion-proof.
Wikipedia may not be "filling up" with things we could have articles about, but it may be filling up with things that unaided new users would guess to write about that will stick.
We could also have articles about every single grid-connected electrical generating plant in the world. As far as I can tell, they are all notable enough. But not many people are interested in writing about many of them.