I've dabbled in trying to do procedural generation in the past (in particular to make good looking trees), and I feel like the missing component (to me) was easy ways to connect geometry. It's easy to make two cylinders, but then trying to join them together is very hard.

In theory CSG can fill this niche, but it can be a bit difficult to think about the problem in that way (for me, at least) since you can no longer view it as just for looping to add vertices, but you instead need to model everything as a 3d shape.

I tried building some routines to take two loops of vertices and connect them through heuristics by adding faces, but I had a hard time getting it to work well (choosing which vertices should connect was much harder than I expected, it was easy to get really bad looking connections).

What I really want to do someday is make a game where there is a system for different modular procedural generation systems to work together in a way that allows for spontaneity. So e.g., systems could "claim" slices of the world, delegate parts of it to other systems (e.g., put some plant here) and connect with other adjacent things.