I'm a little curious to know if there are any mods for Oblivion that restore cut content - stuff that was left incomplete and unimplemented in the game. How much unfinished content was even left in Oblivion?

apart from Sutch mods and I guess kvatch rebuilt, I think nothing ?

Unused content... well, not a lot of it is actually useable, there's just the indication of plans that were never brought to fruition (like Sutch). I remember there being some dialogue (that's removed by the Unofficial Patch) that's supposed to point you towards starting a quest that does not in fact exist.

There's also unused dialogue, either just left out, or with errors in the conditions which mean that it's never actually spoken. There are a few mods that make use of it, Migck's http://www.nexusmods.com/oblivion/mods/44862/ seems like the most complete one.

There's also various technical things that are present in the engine and not made use of, game settings and the like, but I wouldn't really class that as "content" that can be restored.