So I just want to know which Npc Guilds you prefer I also have a few questions.

1. Can you join multiple Guilds?

2. What are the pros and cons of each guild?

3. Are they mandatory towards the main campaign?

Yes

No cons

No

That was your whole list?

What about the other guilds?

Im pretty sure these are the only guilds they are introducing when the game releases. Im sure they will add more later on, but if im wrong correct me because this is the only subject I haven't really looked into. Also If you decide to become a werewolf can you still join the fighters guild.

I think the OP was only posting about the guilds you are confirmed to be able to join? Not sure though. I would prefer the Theives Guild, mainly because its easier to justify my kelptomania that I'll develop when playing this game...

A secret vampire guild would be awesome. They could advertise theirselves as a high society closed club, and only members know the dark truth about it: they are bloodsvckers

I think this is more for singleplayer games though...

Yes but it will still be in ESO so I wonder how they are going to make it work?