I mean just the core game and expansions.
For example i never knew you could kill the dagoths in sixth house bases to wake up sleepers in towns and get rep for that.
Never did tyhe shortcut in the main quest by being over level 20 and over 50 rep to skip 4th and 5th trails to be nerevarine. And i have yet to do the imperial cult and only found half the artifacts.
I think it would be interesting to see what people didnt do in the game or what has been missed.