1. There could be two factions out of civilization who practice the art of summoning, one would summon daedra and the other the undead, they would not like each-other.
2. There could be two nobles or highly positioned persons in a town both intending to take full control over the town through diplomancy (rallying people to your cause) and ofc you could not help both.
3. There could be an arena with two or three factions that require loyalty.
4. There could be racist areas where you are excluded by default if you are a certain race.
5. There could be some sort of monster factions (perhaps goblins VS ogres) where you would join the cause of one or the other (quite possibly shamans or other high ranking fellas of each race could have the ability to speak).
6. There could be fishermen and pirates to the north.
7. There could be factions dedicated to the old Lord Nerevar (dunmer) and the Champion of Cyrodiil (imperials+nords) who would fight each other in vain and childish battles consisting of finding facts about their old heroes so that they can boast about their accomplishments to each-other in some petty "my dad is bigger/stronger than your dad!" fights (we all know them from our childhood and from looking at the world of sports "football team X is better than team Y cos they won tournament Z this many times!").
8. There could be two factions for thieves, one that is greedy and self serving and one that is the classical Robin Hood archetype, and if you join one you gain the disdain of the other group.
And I can go on and on.... so many options, all based on how your character is... too bad Bethesda can′t hire an entire nation to make their games, would be nice to have like 500.000 good programmers making the best game ever but that′s not gonna happen... ever D:
But do imagine the possibilities !