ZDoom

Also, maybe I'm wrong, but this seems designed for game where you can slap a bunch of mods together and just let the game run them all. ZDoom ... doesn't exactly do that, not outside of mod combinations that either don't conflict the slightest bit or have been specifically designed and coded to have forced compatibility.

Mod.io is run by ModDB, which is popular, but, well, I'll just say there are mixed opinions about them and their community management skills circling around here.

That being said, I'm not opposed to a formal community mod manager tool - Quake Injector is great, albeit Java-based - but I'd agree that such a thing is out of the scope of the engine itself.

I'm having a heck of a time trying to find any docs on mod.io's site about what the experience is from a modder/user side. Everything seems to be geared towards "hey, use our API!" without much explanation on how to do things.

From a cursory glance, it looks like it's mostly trying to follow the Steam Workshop model of "subscribing" to a mod -- i.e. the user hits the button and it gets auto-applied to the game on launch, which isn't really a great fit for how Doom mods work.

It does look like there's a separate "download" button though, and there's built-in version management, so that's something. Maybe.

The way I see it, it's no different than what ZDL does, except with a downloader attached as well.

I'm not against the idea behind the feature, but this is something that would have to be done by a third party in the form of an external utility.

I'm not dead set against it, but I have my doubts about usability here. It's the mod makers who would ultimately decide.

There is not a chance this will happen without the aid of external tools and those are well beyond the scope of GZDoom, itself. You may want to explore our Software and Ports forum and the Editors / Asset Manipulation forum for examples of how tools are typically made to work seamlessly with GZDoom, itself.

With the abundance of languages and programming platforms available it should not be hard to find someone who's willing to integrate with the mod.io API and do what you are asking, here.

Any chance for GZDoom to have an integrated mod.io support?

Mod.io is basically a free and open source mod manager similar to Steam Workshop. Besides subscribing to a mod for installation or updates the user has an option to manually download the file.

Knowing the Doom's legacy with mods and maps, this can potentially make running mods much easier without installing any frontend.