Allow Datapacks to add new blocks/items that are variants of existing ones

Something I feel would be very powerful for both map makers, as well as the mojang content team would be the ability for datapacks to define functionally equivalent variants of blocks, or parametrically tweaked versions of items. For example, the ability for a datapack to define something like marble, which is just a regular mining block, but needs to use gold or above to get the marble, otherwise drops something like cracked marble. cracked marble is similar to 'iron ore' in that it does nothing unless used in a craftng recepie.

So far as parametrically tweaked items - it would be nice to define an item - such as a marble shovel, which is essentially a shovel, but has a different durabilty value, possibly a different mining level, and has a different set of enchantments it can get by default. Another option would be something like a gold fishing rod, made using iron nugets and string that has a higher base treasure.

Another idea would be to make 'new' versions of entities which function the same, but have a different outer texture/model - such that you could make a stone chest - which is just a regular chest, but when you combine that with things like new ground blocks, and additional biome generation etc - you can expand the look and feel, as well as the exporability of the minecraft world without need to make engine changes - especially if you can define custom 'mining tools' and 'mining levels' for these blocks, items and entities but staying within existing functionality.