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
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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
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Another interesting thing is that WotS3 use Gamebryo, just like Oblivion.