Barbare
I just realized something according to what I know Mace Tyrell would be 8 years older than Ned Stark that would mean that now he would be a child and with the premature death of Luthor Tyrell Olena would dedicate himself more to trying to annex or at least maintain the power of The Tyrrells to the maximum, I mean I don't see her trying to manipulate more players at the moment, since she herself is in a dangerous position. What I think she would fear in that situation is another realm becoming hostile to her. I think William is more of an opportunity than anything else. Maybe marry him off to one of his daughters. I must say that it is one of the best options since Mace will choose a powerful ally in the future when he is an adult, in addition to Mace himself marrying the second most powerful house in the domain.