Skyrim Extended Cut is an in-progress mod that aims to remix Skyrim's main questline, with promised inclusions like "expanded third act, Dragonborn DLC main quest integrated with Skyrim main quest, more characterization for Alduin & Miraak, more Thalmor involvement, Blades faction overhaul, more stuff to do in Blackreach, mostly new core cast, focus on player agency, guild and side quest completion impact on main quest", and so on. It's intended to make the storyline at the actual center of Skyrim worth engaging with, the whole dragons-and-shouting thing so many players ignore in favor of sidequests.
While work on the Extended Cut continues, a separate sub-team released an example of the kind of thing we can expect from it—only instead of the main quest, they've focused on one of the Creation Club add-ons bundled with Skyrim Anniversary Edition, Saints & Seducers. The biggest Creation in terms of stuff, it added quests, creatures, weapons, armor, and a tileset all referencing much-loved Oblivion expansion The Shivering Isles.
Like most of the Creation Club additions, Saints & Seducers involved NPCs who suddenly became shy, communicating via letters and books so new dialogue didn't need to be recorded. To begin the quest you had to track down one of the khajiit merchants travelling back and forth between Whiterun and Markarth, who then handed you a piece of paper like he was sneaking you a note in class. After enough repeated scenes like that, the Anniversary Edition stopped feeling like it was worth breaking half my mods to install.
The Extended Cut version of Saints & Seducers has a radically different questline, beginning dramatically with an earthquake in the city of Solitude. You investigate the sewers where the quake originated, finding a portal to to part of the Shivering Isles called The Asylum and meeting the first of several fully voiced NPCs.
Here's where I had my doubts about the Extended Cut project. While a handful of mods like The Forgotten City have shipped with high-quality voice acting, plenty of others struggle with uneven recording quality, characters who sound jarringly modern, and a general out-of-place-ness that prevents them from feeling like they belong in Skyrim.
Thankfully, that's one of the places where Saints & Seducers Extended Cut is at its best, with multiple talkative characters who sound like they belong. Even the voice actor playing the madgod Sheogorath (well, a shopkeeper whose name is an anagram of Lord Sheogorath) nails the character's abrupt pitch shifts and drift between Scottish and Irish accents. The new worldspace is impressively accurate too, resembling a slice of The Shivering Isles as it looked back in 2007, all giant fungus and glowing puffballs.
It really shows what a mature mod scene is capable of. As well as incorporating Creation Club add-ons (when installing the mod there are options to shift other Creations like the magic weapon Shadowrend to The Asylum, where it becomes part of a sidequest), Saints & Seducers Extended builds on existing mods. Returning Shivering Isles monsters, including grummites and flesh atronachs, come from the Mihail Monsters and Animals mods, while a bust of Sheogorath comes from a mod that's part of a series adding Daedric Shrines to Skyrim, and several other assets are borrowed with permission from existing work.
That's given me more confidence in what the full Extended Cut will be able to achieve. Though an ambitious project, its subject is the heart of Skyrim, part of the game that's already been resculpted in more piecemeal ways by plenty of other mods. Where Saints & Seducers Extended finds room for an appropriately quirky existing mod called Merlin the Corgi (a magical dog follower who can be recruited if you have both mods installed), the full Skyrim Extended Cut will have years' worth of overlapping mods to borrow from and collaborate with. The trailer already sneaks in a cameo for popular mod follower Lucien, and apparently care is being taken not to contradict the world-expanding of Beyond Skyrim.
I had several fun hours playing this sequel to the best expansion Bethesda's ever made, and it convinced me to try the full Skyrim Extended Cut whenever it's finally finished. It sounds ideal for the kind of vanilla-plus playthrough where, rather than completely changing Skyrim so it looks like anime and plays like Dark Souls or whatever, you refine the stuff that's already there to make it more to your taste.
One downside to Saints & Seducers Extended is that it recommends starting a new game before you try it for the sake of stability. (Even doing that, I had to re-load a save to fight the final boss a second time when the NPC I needed to talk to afterward fell through the floor.) It also requires a player-character who is at least level 20 and has completed the Mind of Madness sidequest in Solitude.
I recommend using an alternate start mod like Skyrim Unbound Reborn or Live Another Life to skip the tutorial and set Solitude as your starting location, then use Skyrim's console commands to level up. You can either improve your skills with player.advlevel [skill] [#], where skill is the name of the skill (although the console seems to be based on an early build so archery is called marksman and speech is speechcraft), and # is the amount of experience you want to add to it. For example, player.advlevel speech 9999 should give you enough speechcraft to get you through Mind of Madness. If you want to fine tune, the console command incpcs [skill] will push whichever skill you choose up one level at a time.