I ran this suggestion through Drullkus, this is what we came to the conclusion of;

In pack.mcmeta, you'd be able to provide a link to an external .json file somewhere on the web (e.g; github) that stores the recommended version and the latest version of the pack.

In pack.mcmeta, you should also provide the resourcepack's version itself, to be able to compare it to the external .json's

Thoughts?

Specification:

Root compound
├─ pack: Vanilla. For data about the resourcepack.
└─ ctm: CTM-specific data
‌ ‌ ‌ ‌ ‌ ‌ ‌ ‌ ‌ ├─ version: String version. Should follow semantic notation of major.minor.build
‌ ‌ ‌ ‌ ‌ ‌ ‌ ‌ ‌ └─ update_json: URL to external json file. Should probably mimic Forge's json format.

{
  "pack": {
    "pack_format": 2,
    "description": "testing pack for AO"
  },
  "ctm": {
    "version": "1.4.3",
    "update_json": "https://raw.githubusercontent.com/TeamTwilight/twilightforest/1.12.x/update.json"
  }
}

Seems very out of scope for this project, though I could see it being useful in another mod.

I picked this mod to make this suggestion because it's one of the two or three mods I see resourcepackers using a lot.

Making the updater it's own mod would flop almost straight out of the gate, rip.