[Suggestion] Suggestions, as a pack painter.

[ghost]

1 - Overlays: A better implementation of what "Edge" was supposed to be.
Example:

2 - Conditional Item Textures/Models: Item models, or textures changing depending on certain NBT data. E.g; name, enchantment, stacksize, etc.
(Also, having elytras and armour change depending on these factors would also be cool, but it's not as needed.)

3 - Conditional Block Textures/Models: Model, or texture changing depending on certain NBT data. E.g; name, disk, if certain items are in the block's inventory, adjacent blocks, biome, etc.

4 - Conditional Entity Textures (or models); The texture (or model, if possible) changing depending on certain NBT data. E.g; name, biome.
4.1 - Random Entity Textures (or models); Self explanatory, randomized textures for an entity, or model, if possible.

(If it's only possible to override a tile entities's model, that's perfectly fine.)

[ghost]

4.2 - Emissive entity textures, because why not.

[ghost]

(Less serious) 5 - Custom sky: Ability to change the properties of the sky.
(Less serious) 6 - Custom Fog: Ability to tweak the fog colours per resourcepack.
(Definitely less serious) 7 - Custom Lightmap: Ability to tweak the lightmap, especially per dimension.

[ChloeDawn]

Regarding the last requests, you can refer to this ticket asiekierka/MageMC#1. The current releases for MAGE can be found on Curse, here.

[tterrag1098]

1. This can mostly be accomplished as is, see CTP's obsidian
2. I've considered adding more property getters for items but this is difficult to do in a way that lets it still work without CTM installed
3. Same as 2
4. CTM doesn't touch entities at all and I think doing so would be out of scope as entities do not use JSON models nor respect any kind of texture metadata.

[ChloeDawn]

1. Isn't really possible without a lot of hacky stuff. It's possible though. Would need to asm IBakedModel#getQuads, and use the extended states of adjacent blocks to determine if an adjacent block should be drawing an overlay on the block model. The dirty workaround for now would be proxying every model in your resourcepack to an "edges" texture type if the "target" block is present; but I wouldn't recommend that (it would also only support a single overlay, so it's pretty useless as a workaround). Ignore everything, I misunderstood OptiFine's implementation.