So while this thread is temporarily re-animated...
Imagine what biome mods could do for us if we had
cubic chunks instead of linear ones. Quasi-infinite world height without any performance hit. Take a look at
this video (not Minecraft, but a custom engine that looks very similar) to see what it would feel like to have mountains rising over one thousand blocks above ocean level. For some strange reason, this idea used to be all the rage a year or two ago, but has since been forgotten. There even used to be a mod for this while Minecraft was still in beta, but it too fell into obscurity.