One thing that bothers me heavily about Oblivion, is that you're forced to be a goodie-goodie two shoes, and fight alongside all of the Tamriel born people against the Daedra, something that would make me like Oblivion much more, would be something that very heavily alters the main quest, so that you can choose to side with the Daedra... Sure it may seem like a horrible act of betrayal, maybe even a terrible idea for you in the long run, but regardless, I still wish the idea was there.

Is there any mod for Oblivion's main quest, which I don't know about, that LETS you actually side WITH the Daedra whom are invading it? And you know, with it's own indepth questline and all that?

Your bet is Servant of the dawn, Google it.

It has only one drawback, but it is a huge one: you must complete the main quest first, then you can play it.

So you must be the goodie once again.
The idea behind it is that your character redeems himself after that, so to say.

Whoa, that mod is fascinating, I'm surprised I never heard of it before.

It is said to be one of the best Oblivion - or entire TES series, for that matter - mods out there, so you can't go wrong with it.