What would have happened to a ward like Theon Greyjoy in “ordinary” circumstances (no Others, Varys/Littlefinger/Lannister conspiracies)? He was 20 at the beginning of the series – was Ned going to keep Theon until Balon died, even if Balon lived another 30 years? Or at some point would Ned let Theon get married and leave Winterfell? Just return Theon to Balon? Married Theon to Sansa/Arya? Done something else? How does a situation like this usually end?

Well, keep in mind that he’s both a ward and a hostage, which complicates the normal routine, which was generally to return people to home upon majority.

However, one of the things you did see happening with hostages was an attempt to reinforce loyalties from childhood/education by having them marry someone from their captor’s region – I seem to recall a bunch of Scottish hostages in the 16th century were married into English families to give them a stake in good Anglo-Scottish relations once they were returned home.