A world height increase is not feasible due to the way worldgen in 1.7 works and how mods generate world elements. I could make a laundry list of reasons but I guarantee you it will never happen, and any mod that does so would require every mod to add compat (which many dead ARR mods will require patches for)
Furthermore I find the new height completely unnecessary and makes caving annoying. There were plenty of ways to make the world interesting without adding more needless space that modern doesn’t even do much with, leaving caves that are huge and empty, and in my opinion, just worse. There aren’t enough biome and there’s not enough new cave features to make 1.18 caves interesting, it’s just night vision and finding the overly common huge caves to farm diamonds. BORING