There should probably be a menu of sorts which would be used for customising certain features about worlds which fall out of the scope of CustomCubic but are still considered important to Cubic Chunks worlds:

• #460 Height of the world (default -1073741824 to 1073741823)
• Possibly also a way to customise the x and z axis limits of the world, since they're currently capped at 30 million. May need to come with warnings about weird server desyncs in multiplier and the like though.
• #457 Sky colour by height (indev-level sky and fog colour customisation would also be interesting)
• #143 Cloud height and potentially cloud layers (may lead to incompatibilities with optifine) (maybe cloud colour could also be changed like in indev?)
• #167 Void skybox options of some sort
• #478 Height limits of the Nether and End (currently both capped at 127 but could be toggled to 255)
• #459 A tab containing all mobs which only spawn within certain height ranges, so these ranges can be configured

#460 Height of the world (default -1073741824 to 1073741823)

Agreed, this needs to be done

Possibly also a way to customise the x and z axis limits of the world, since they're currently capped at 30 million. May need to come with warnings about weird server desyncs in multiplier and the like though.

No for 1.12.2, there are commands for that. Will be needed for 1.14+.

#457 Sky colour by height (indev-level sky and fog colour customisation would also be interesting)

This is beyond the scope of cubic chunks

#143 Cloud height and potentially cloud layers (may lead to incompatibilities with optifine) (maybe cloud colour could also be changed like in indev?)

That may eventually be an option, but unlikely.

#167 Void skybox options of some sort

If anything, I would just remove void-specific skybox completely as it's completely unnecessary.

#478 Height limits of the Nether and End (currently both capped at 127 but could be toggled to 255)

With it being part of CubicWorldGen, this would become part of worldgen customization

#459 A tab containing all mobs which only spawn within certain height ranges, so these ranges can be configured

For something like slimes - agreed, would be useful., but only on 1.12.x.
For 1.14.x, it will be server config (which is per world) using standard forge mechanisms.

Something else which would be useful: some way to customise brightness levels and light maps. The nether and end are not nearly as dark at light level 0, which definitely results in some different aesthetics coming into play, which the customisation of could be a useful tool for painting certain worlds.

I think this is a feature optifine has for resource packs.