Have you even played it?
It's more than just a mod. It's a game that surpasses Oblivion in pretty much every way possible. Much more content, a completely hand-designed map, etc. Simply put, it's the best RPG of the last decade.
I didn't really like it that much. Very impressive for being a mod, sure, but after 10ish hours of exploring I was rather bored of it. The story didn't interest me at all. The capital Erothin did impress me a lot though, compared to the relatively plain towns of Oblivion.
As for other mods, my favorite mod of all time remain to be http://www.thief2x.com/, which very much feels like an official Thief 2 expansion
That sounds like a very interesting game, I'll look around to see if I can find it. Is the story linked to the previous games, or can I just jump in at number 3?
The games storys are separate and there are no recurring locations or characters either. They just have a similiar premise and gameplay. They are all fairly short, but offer a lot of choices and many different endings, so there are many things you can try differently if you try it a second time.
Be warned though, the gameplay can hardly be called polished, so some oversight is needed. Don't expect the same movement fluidity as in Assassin's Creed or anything
Another interesting thing is that WotS3 use Gamebryo, just like Oblivion.