During the quest 'Dagon Shrine' in which you go undercover to infiltrate the Mythic Dawn and steal the Mysterium Xarxes, the player is forced to be the good guy once again. This would have given the player a real choice in the game and not be forced to be the good guy the whole time. Bethesda could easily (well, not easily, but you know what I mean) have added another questline in which you could join the Order of the Black Worm and travel through the ranks.

Now, I think the player should have had a choice to join Mannimarco and fight at his side against the Mage's Guild. I already have more servants than I need.'

Mannimarco: 'I'm afraid you misunderstand. Player: 'I'll serve you if you spare me.' Mannimarco: 'I will make you another in a long tradition of Worm Thralls, and take my time in studying you. Before the final battle, however, there is a fairly long exchange of information and threats. In the final quest of the Mages Guild, 'Confront the King,' you find and kill Mannimarco, the King of Worms in his lair, Echo Cave.

Oblivion servant of the dawn