Enderal: The Shards Of Order by SureAI is arguably the most impressive thing to come from The Elder Scrolls 5: Skyrim's massive mod scene. While it may use Skyrim's engine, it uses it to tell its own story in a new world, fully voiced and expertly produced. If there was any mod worthy of a high-profile release on Steam alongside Skyrim itself, it'd be Enderal. And that's what's happening.
While there's some precedent for it (Skyrim Scipt Extender, a mod prerequisite, is available on Steam), Enderal is the first slab of game content to escape the Steam Workshop and establish its own store page. While there's no date set for it yet, SureAI hope to release Enderal with its upcoming expansion on Steam soon, although you can get it direct from SureAI now.
While it shares an engine with Skyrim, Enderal's magic, big concepts and stat systems are largely its own. While I've only had the chance to play a few hours of it when it first released, Enderal almost feels like a strange parallel dimension version of Skyrim. It's a lengthy RPG in its own right with decent voice acting (in both English and German) and a lot of places to explore. It's also a little more uncompromising and old-school than Skyrim, so don't expect to go wildly off the beaten path without the path giving you a beating in return.
As polished as Enderal is, it's taken time and effort to produce something so impressive. Enderal is the latest in a series of total conversion mods produced by the studio. They started out with the German-only Myar Aranath, then the English-translated Arktwend for Morrowind. They later graduated to Oblivion with the impressive and fully voiced Nehrim. Enderal is by far their biggest project, and could easily pass for a completely standalone game if it weren't for its software requirements. They're also hard at work on a major expansion for it, called Enderal: Forgotten Stories.
As an aside, Skyrim mod The Forgotten City is currently being remade as a fully standalone game. You can read more about that here.
While there's no date listed, you can find Enderal here on Steam. Once it's released you'll need a vanilla copy of The Elder Scrolls V: Skyrim, though none of its expansions are required. Until then, you can find all of SureAI's mods here.