A few questions about datapacking...

I have two for now, but I'm only doing like the art, ideas, story and some building.
Are custom dimensions possible in an average survival gameplay, including custom blocks?
Can custom blocks be used in stuff like caves and quartz in the new dimension and be like ores? I have a plan to just use full-on blocks in the new dimension instead of actual ore blocks.
Can a custom mob transform into another custom mob like how the ocelots used to transform into cats when tamed?
I'll try and come up with some more questions while modeling and building in Creative Mode. Also, please feel free to go into DMs if You want to help code for the datapack addition for My Optifine resource pack.