I personally don't think Stacy is a suspect. She has some gaps in her past, as did our MC, and I don't think there's any need to include a betrayal plot because she turns out to be our MC's childhood friend who got him to play basketball. Especially since Damien already had the role of the supposed friend who betrays our protagonist.
My personal favorite continues to be, if there is to be another enemy within our own ranks, Ryker's sister. The mysterious accident in which MC and his father were involved is still ominous and offers much more potential in terms of the fate of Ryker's and Katie's parents.