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.
In a way, she sees William as an opportunity. My take on the subject is that the agricultural powers of Westeros are first and foremost the Reach, then to a very lesser extent the Riverlands and the Crownlands, and that Olenna, as the Matriarch of the Tyrells, would like, at worst, to keep it that way and, at best, to have the entire realm dependent only on the Reach for food. The problem for her right now is that House Tully, as the Great House in charge of the Riverlands is developping abnormally and that if this development were to expand to agriculture it would greatly threaten the power of the Tyrells. In any case, Olenna realized that House Tully had become a significant force and that she had to do something about it, maybe it will lead to her House and William becoming allies, maybe the other way around, maybe neither, you will find out in the next episode of Dragon Ball Z...