Seeing this topic made me realize I've only beaten the main quest once... I restart too many times.
^^ This, I've only completed the main quest once with a Dunmer crusader. There's so much I haven't done, like completed Tribunal or Bloodmoon...and I've had the game for years now. Like the person I've quoted has said, I find myself restarting way too many times to count. So, there's a lot I can still find and do and it would be new to me, because I know I haven't found the vast majority of artifacts or other things in the game, and I know I've only found a fraction of the tombs, caves and ruins in the game, not including the expansions.
As far as a list, I've never completed the following factions to rise to head them:
Morag Tong, Mage's Guild, East Empire Company, Thieves Guild, Imperial Cult, Temple, Telvanni, or all the Vampire quests.
I think I've only ever completed Imperial Legion and House Redoran, and only headed House Hlaalu due to the glitch, not just by playing through it.
As far as vampires, I like playing Altmer Sorcerers, so I always end up an Aundae, but I may start up a new game sometime and play as a nightblade, thief or assassin who joins the Berne bloodline. Speaking of that, I also have the Bloodlines mod, and I've only gotten through the first part of it.