Elite is really primitive compared to today's games, but it had a huge galaxy to explore (even if all the systems were essentially the same 1-star-and-planet), then eventually you find out there are 8 such galaxies. It was fun to find good trade routes between the worlds, risk going to a slightly more lawless planet to beef up your credits or ranking. The emergence here would be the fact that the economic type of a planet was dependent on its seed value, and together with other nearby generated planets created "trade routes". These routes were not coded into the game, they emerge as a property of the way the procedural generation works out.
I think the problem with making so much procedural is that it misses the point that the game should be fun to play. What's the game-play difference in having random trees or dependent on sunlight and rain? In the end, they are just a bunch of trees :-P
Because when things "make sense," players pick up on it and use it to their advantage (and feel very clever). Given a procgen survival game where you are in a desert, trees might only grow near water, so if you are thirsty, you might look for vegetation.
One review damned NMS for lack of player-significant effects, but appreciated the prettiness of some of the results.
There's a property I enjoy, of a kind of global dependence (though mediated locally), instead of isolated randomness. e.g. instead of trees appearing randomly, they are dependent on rain, sunlight, latitude, altitude, soil coarseness and nutrients. And they are dependent on other trees.
This is surely includes under "emergent", but that term seems unnecessarily broad to me.
Rainfall in turn depends on local and wider topography (mountains, evaporation water sources), and wind patterns... which in turn depend on local topography and latitude etc.
Even a river's path is affected by its watershed (and rainfull etc), and all the preceding terrain, especially steepness (higher velocity carries more sediment, which abrasively erodes even faster) and soil finer-grained erodes more than coarser). River change course from time to time due to erosion.
It seems iteration is the only way to capture these dependent effects, but its computationally very expensive. NMS already has very long planet-generation times ("travelling"), so it's probably not feasible for that style of game.
Earth is a computer after all.