Personally, I prefer multiple endings for side quests.
Reasons are -
1. TES games are based on one main history.
2. TES games are about a freedom world to live in.
Since there are too many side quests in game,
the easy way to tell unique story would be optional guild endings.
For example:
Thieves' guild may choose to steal the name of the throne.
Fighters' can choose to kill all good dragons for unique power.
Mages' can learn to summon undead dragons feeding on souls.
Dark brotherhood, of course, may slay all guild masters and replace them with false ones.
(If you are already the guild master, you'll need to cheat Sithis.)
I know all these are not gonna be included.
Just think it one step closer to real role playing, and it's possible for future TES games.
What do you think ?