The "make it beautiful" part involved unifying all these use cases into a single set of abstractions that could express them all concisely, with high performance, and without needing any other infrastructure. Since I was building such a general platform, I was also able to consider use cases I hadn't directly worked on – basically just looking at popular web applications and their features.
Leaving Twitter in 2013 was the start of the "make it beautiful" phase. By that point I had already figured out the broad outlines of what such a next generation platform would look like (event sourcing + materialized views, indexing with data structures instead of data models). It was a long road from there to figure out the details and turn it into a production platform.