I also like Sothron Ambitions: http://towerofthehand.com/blog/2012/01/05-southron-ambitions/ http://youtu.be/-iVvZTOMK0M Essentially, during the War of the Ninepenny Kings, the leaders or heirs to several great and small houses meet one another and develop relationships: Rickard Stark; John Arryn, Steffon Baratheon's Father, Bryden and maybe Hoster Tully, even Petyr Baelisch's Father.
After war, Rickard Stark betrothes his heir Brandon to Hoster Tully's daughter, his daughter Lyanna to Steffon Baratheon's son & Ned's friend Robert, and sends Ned to foster w/Arryn. Steffon Baratheon sends his heir, Robert, to foster at the Vale w/Jon Arryn & agrees to marry him to Rickard Stark's daughter. Hosted Tully even agrees to take in Petyr Baelisch as a fosterling. All of a sudden, Stark (North), Arryn (Vale), Tully (Riverlands), and Baratheon (Stormlands) have tight relationships, perhaps tighter than any relationship with the Targaryens.
ALSO, had Joanna Lannister not died giving birth to Tyrion, Oberyn & Elia Martell would have been betrothed to Cersei & Jaime Lannister, tying Martells to Lannisters. If Elia Martell were married to Jaime Lannister, She would not have been able to Mary Rhaegar, setting stage for fiasco at Harrenhall Tournament. If Rhaegar did not wait to marry his sister Daenerys, he would have been forced to marry a Tyrell, maybe Mace's sister Mina or Janna.

If Rhaegar still felt need to crown Lyanna Stark the Queen of Love and Beauty at HarrenHall and rul off with her, the same alliance of Arryn, Baratheon, Stark & Tully would have rose up against the Targaryens. Lannister & Martell would be forced to choose between keeping the Mad King in place (the man that Tywin quit on and who took inappropriate liberties with Joanna at his wedding) or support the rebels and their numbers. Only the Tyrells would have been left to stand w/the Targaryens, as they did