Right now, pack-makers can manually create JSON files to represent ore distribution, which is awesome.

But for some heavily modded packs, the information given by JER can be either misleading or non-existent. For instance, let's say that using CustomNPCs, I have created a mob I call a "Blood Zombie", which drops a rare item defined using Content Tweaker called "Bloodstained Robe". Let's then say a player runs into a crafting recipe that requires a Bloodstained Robe. They click it in JEI... and get nothing.

Obviously, its completely unreasonable to expect your mod to identify this. But if you added craft-tweaker support, you could punt this responsibility back to the mod authors and make them do it.

For instance:

mods.jer.JER.addMobEntry("Blood Zombie", <entity:customnpcs:customnpc>.withTag({Texture: "customnpcs:textures/entity/humanmale/woundedsteve.png"}))
mods.jer.JER.addMobDrop("Blood Zombie", <contenttweaker:bloodstainedrobes>, 0, 1, false);

Could display the given entity, with the given name, with the given drop.

This would make it far easier for pack makers to help players find new or modified items in intuitive ways. If a player, for instance, receives a quest to get a bloodstained robe, they often won't read the quest description; instead, they'll just go to JEI and look up the item in question. This sort of functionality would make it so a prepared modpack author can work with that player expectation instead of against it.

Similar features could be put in for all sorts of JER handlers. For instance, trade information can be manually entered so a pack author can help players find CustomNPC vendors that sell items in an adventure map. Authors can give JER data for their Recurrent Complex structures, so JER can display rare items that appear in pack-author-created locations. Meticulous pack authors can add crop data for mods like immersive engineering or natura.

This would be amazingly useful.

Ah, forgot to mention in the original post:

If supporting CraftTweaker isn't your thing, you could also achieve the same functionality by loading user-created JSON files. For instance, the example above could be represented by the json:

{
	"newEntities": [{
		"displayName": "Bloody Zombie",
		"entitytodisplay": "<customnpcs:customnpc>",
		"entitydisplaynbt": {
			"Texture": "customnpcs:textures/entity/humanmale/woundedsteve.png"
		},
		"drops": [{
			"item": "<contenttweaker:bloodyrobes>",
			"min": 0,
			"max": 1,
			"affectedByLooting": false
		}]
	}]
}

Just trying to provide as many ideas to achieve this functionality as possible, in case one of them sticks. ;)