In past TES games you could kill the Dark brotherhood, but in ESO that won't be possible, so do you feel like they are going to add a new guild (Like the new guild Undaunted) for players that which to oppose the brotherhood, since you can't just slaughter all of them when you meet them.

That would be speculation for waaay in the future. DB/thieves guild isn't in at launch.

This isn't new, I Know that, but I am just saying ESO already showed they are going to add new guilds Example "Undaunted" so do you think we will get a guild to appose the Dark brotherhood for players that do not wish to do anything with the night mother?

I do think it's a great idea!

I don't think a guild should be created for the sole purpose of having an option for players who aren't interested in it. If there were a lore established reason to create one beyond the scope of just being opposed to the dark brotherhood for the sake of a dichotomy then sure.

For example maybe the hypothetical guild could have quests that send you to protect high value assassination targets? but really who even likes escort quests?? Maybe you could 'purify' (assassinate) members of the dark brotherhood in the name of some other deity lol!

But just for the sake of having opposition? Not in my opinion

The Morag Tong would make sense as a rival factions. Though the Morag Tong aren't a Tamriel wide organization.

They shall fall by our hands.

The only guild that really opposes the Brotherhood is their parent organization, the Morag Tong.

Well we don't know how the Morag Tong will be in this Era, idk they could be really big organization from what they have been in previous times.

not in all games though. in Skyrim I remember you could destroy their sanctuary,but not in Oblivion.

Well killing Lucien pretty much stopped them, since he dropped the Blade of Woe, where players that joined the brotherhood would get it later levels.

Then there should be opposing guilds to fighters, mages guild to.

Why only DB?

and as earlier stated no rival guild known.

The DB has questionable motives though, not everyone wants to be a bad guy and kill the innocent just because another person wants them dead and is offering coin.

No one is forcing you to join. I will not join the undaunted, the fighters guild and mages guild with my first char. Only if I would want their skill set.