Yes you are right any royals all throughout history married coz they would get something- title, money, land or ego boost.
Spoiler The first born Joffrey is technically not the first born. She had one who was not allowed to be born. Myrcella's death was also slightly of because of Cersei. I don't understand why she didn't stop when Tyrion gave her away. The last one, no she didn't see it coming. The consequence of bringing children into the game sadly.
I will be happy if its extends to another season that way. There is also valyrian steel and dragonhorn for the WW and dragons.
Spoiler Jon and Dany could be siblings. Jon could be Lyanna Start and Rhaegar Targaryen's baby as per the last season. So Jon may be half Targaryen and half Stark. Even then a marriage will not be surprising given the way GOT is.
I don't think they will. If they will, I want to see Spoiler the infamous scene where Rhaegar gives Lyanna the winter roses.