TL;DR
With the DLC, if you turn off the "Space age" mod = lose access to a lot, so the ideea is that instead of visiting different planets, the game should generate large distinct regions on Nauvis that replicate planetary environments like Vulcanus, Aquilo, Gleba and Fulgora so we still have access to everything (except space platforms and related, with rockets only used to farm asteroids).
What?
I’m suggesting a region-based system where planetary environments are integrated into a single, continuous Nauvis map. For example, a Vulcanus region would be an entire area filled with Vulcanus terrain until the edge of that the world map. Another region could be Aquilo terrain. These regions would each simulate different planets as their own massive areas within the same world. You just keep moving across the map to enter new biomes.
Holmium should be found as a normal ore, just like any other resource, with thunder happening as a weather effect—something new, since Nauvis by default doesn’t have any weather system. And of course, the more you advance the worse it becomes (similar to Vulcanus initial area being save, so you have a "save" area from thunder where you start, but get worse as you go further from center)
Why?
Because the mods that try to do this already don’t do it well—they end up being messy, randomly mixing different planetary elements in a single area (Nekeheaven (mod planet) is by far the best exemple of this), and they suffer from compatibility issues with other mods.
If this was part of the base game, it would be handled properly and allow for a visually clean and consistent experience. It would eliminate all the weird terrain and biome overlap and create a better, more immersive system for integrating planetary content. Plus, other mods could then rely on this region system instead of trying to hack it in on their own.
As like this, all units, buildings, items, and pretty much anything except for space platforms and things related to them are still available.
And this would make train world playstyles amazing.