Is it possible to complete this game without joining the Dark Brotherhood, and becoming a thief, Vampire or Werewolf?

and if so how much harder will the game be with these restrictions?

http://www.troll.me/images/boromir/one-does-not-simply-finish-skyrim.jpg

There is no right or wrong way to play Skyrim, friend. You can make the game as hard as you wish in the options menu.

Not sure what you mean by "complete the game". As pointed out in another thread, the game does not end.

You can complete the main quest without becoming a werewolf, vampire or joining the Dark Brotherhood. You can play for thousands of hours without ever becoming a werewolf, vampire or joining the dark brotherhood. The game is not particularly hard to begin with so you can play it as you like.

But, if you consider "completion" to mean unlocking every achievement or completing every quest, well obviously you cannot complete all of the dark brotherhood quests without joining them although There are also quests related to becomming a werewolf and the Dawnguard DLC has quests related to becomming a vampire, so you cannot complete those quests unless you become werewolf or a vampire.

If you mean completing the main quest, yes. You don't have to join the Brotherhood or the Thieves Guild or be a werewolf(Companions), or be a vampire. I usually don't do any of these and I don't think it makes things any harder.

If you mean doing all of the major quest lines, no.

Skyrim, for me, just.... exists. There aren't any "must do" quests. Every quest, main, side or otherwise, is taken on with the strict limitation of it being something the given character would do, pending their personality profile. In other words... not every character does every quest.

In fact, I have yet to fully complete the MQ and have never even started the CW questline and this has been across almost 60 characters and 3 years.

If you mean completing the main questline, it's independent. As for the guilds or being a werewolf/vampire, they certainly offer some benefits -but none of them is considered a must have/be, for completing the main quest.

Speaking of the game as a whole, it certainly isn't just about the main quest. There's certainly much more to do and being the open-ended game it is, it can end now because you said so -or never, again because you said so

The ones you mentioned are the ones you don't have to do. But you can't complete the main quest without doing parts of the College of Winterhold quest-line, as you need access to the college itself. The main quest also steers you toward joining the Thieves Guild, but but you can ignore that, as you're not locked out in that case.

The game is designed in most part to allow the player a free choice of quests, if not their sequence, so those linkages are a sore point for many.

Can you adapt a male child without joining the Dark Brotherhood?

Whoever told you that only members of the DB can adopt male children, was wrong -or a liar.

I think you meant without killing Grelod the kind.. (you dont need to join the db, just kill her to adopt any child in honorhall)

but if you dont want to kill her.. you can adopt Alesan (dawnstar) and Blasie (solitude stables)

http://www.quickmeme.com/img/a5/a5ceb934249dc824539daf66685a9f4285361254ab1b656496aac5565b2f610e.jpg

Couldn't help myself But in all seriousness, you could play the vanilla game wearing nothing but your birthday suit while punching dragons to death with your bare hands. It all comes down to how you play the game, and which difficulty setting you're on.

Yes. http://www.uesp.net/wiki/Skyrim:Adoption#Adoptable_Children

You can take any old route you like with Skyrim, that's the beauty of it. Completion of the game is a bit of a grey area though and purely down to each player for their own definition of it.