#1

I’ve seen, that there is a main mission to complete.

 Whats about the side missions which are independent of the main mission?  
 Are there some, what can we do if the main mission is completed?  
 How open will this game really be?  
 What opportunities will we have?
#2

Good question, I’m often the type of player who instance goes running of doing what I want, rarely doing the main quest. For example I have around 100 hours of skyrim, but I havent even played the main quest except the “get dragons” part of it.

Would like to know what is possible here too, without doing the mainquest

#3

I believe they said on their Reddit AMA that the game will be playable after the main quest was completed. Someone else may be able to confirm for sure, though.