Open Cities, it puts all the cities except the IC and Kvatch in the main worldspace. Once you try it you'll wonder how you ever played without it.
There are two variants; Classic keeps the original city layouts, and Reborn changes them to be more logical and closer to the concept art. I personally like Reborn a lot, but you might want to go with Classic as it sounds like you're looking for an experience closer to vanilla.
http://www.tesnexus.com/downloads/file.php?id=16360
http://www.tesnexus.com/downloads/file.php?id=33003
There's also Better Cities, which really helps to make the cities feel unique and like real renaissance cities, but it does come with a performance hit.
Integration: The Stranded Light is very fun to me, and personally for me it does the opposite of going too "out there." It makes a lot of Oblivion look like some lame action game that was trying to be an RPG. It has skill checks, interesting stories, well developed characters, multiple solutions, recogmiotion of race, achievements, stats, etc. proper integration into the game world, lore from the elder scroll series which Oblivion refuses to acknowledge in favor of a more bland 'black and white' world, and just so much more.
It makes a lot of the vanilla quests look like poor excuses for a western RPG while the Integration quests far out shine them.
There is one problem here, though: the backbone of this mod is omnipresent and impossible to fully remove. Being based on RBP and LAME, two mods that drastically alter the gameplay, (so far as I have heard) even the requirements-free version without those two will not be able to fully preserve the vanilla experience. I've noted a few people already complaining about the fact that all of the races in RBP are still there and still playable, for instance. This might be too much for the OP's liking, but if he feels inclined, I certainly will give my endorsemant.
Other mods you might like include:
http://tesnexus.com/downloads/file.php?id=18305 A robust and realistic weather experience that includes things like being able to see through building windows and improved interior and exterior lighting.
http://tesnexus.com/downloads/file.php?id=20218 Adds new roads and bridges around Cyrodiil to make traveling by foot a bit less of a headache.
http://tesnexus.com/downloads/file.php?id=31668 Makes the cathedrals to the Nine throughout the game's cities feel much more distinct and a lot less generic.
http://tesnexus.com/downloads/file.php?id=39804 Lots of new audio for the game to help svck you into the world. You can hear horses whinny at the stables now, or hear the roar of the ocean in Anvil harbor, to even little rowboats bumping up against their moorings.