I am a lightweight. I have over 100 hours and i still did not scratch the main quest. I have a couple of mods that i considered useful, but not too many. Also only around level 55.
But to be honest...Oblivion definitely had better quests. And Morrowind it's still my favorite (juggling the politics between various guilds and houses was a pain in the arse, but i became the big cheese in all the guilds through careful navigation). My house of choice reflects my love for Thieves/Assassins and that was Hlaalu.
God, i so miss the Morrowind's weirdness. Plus, we had a lot of quests linked to the Empire (Blades, Temple and Imperial Legion). I cringed seeing what a joke the Empire became in Oblivion and Skyrim. Ruined forts, a non-presence in the main province of Cyrodiil....a Temple that barely manages to keep their faithful around. Meanwhile, the daedric Gods get more and more adepts. Also, the Blades being replaced with Penitus Occulatus, a faction that seems to be lost in the background....Morag Tong replaced with the Dark Brotherhood (Bethesda should have put both in the game and put you in the position of even destroy one of them or both or mending the rift between them and reuniting them). Knights of the Nine from Oblivion disappeared again and i doubt that we'll ever see them in another game.
Bethesda really should learn to better use the lore that they put in the older games better. Less fetch/kill quests, better faction/secondary quests, as it was originally. Skyrim wears your will to play because of the sheer number of kill quests. And removing the level cap was a bad idea. It took away what it made the chosen race/character yours.
Back to GOG...I'd be more than happy to see Oblivion and Fallout 3 here. Those are the most likely to happen sooner than later.