I think that Zed and Syndra might ally with Noxus. Syndra respects power (primarily her own), and Noxus respects power. Zed wants the freedom to teach his shadow techniques; Noxus doesn't shy away from controversial methods of developing strength.
I think Yasuo would indeed return to defend Ionia.
Riven probably wouldn't completely betray her home country. She wants Noxus to become more "respectable" and "conservative," as opposed to new age Noxus using horrific abominations (
) and controversial, terrible chemicals (
) to reach its ends (plus there's the whole background political power play going on with
,
, and the Black Rose, which I doubt Riven cares for). She's faithful to Noxus itself, but she refuses to fight alongside such unsavory characters.
Wukong would probably help Ionia, for the sake of his teacher (and because he seems to be a fundamentally "good" character that would see the invasion of Ionia as unjust). However, a fight with Noxus is not necessarily a fair challenge (the sort of thing Wukong sought by joining the League)— he might enjoy a fight with Sion or
, perhaps
, but
, Swain, and LeBlanc would probably seek "unfair" tactical advantages over him (trickery/deceit, ambush, overwhelming force for fighting Wukong alone, etc.). Singed probably wouldn't provide a fun fight, either, seeing as he prefers to use poison rather than his own combat skills to fight his enemies.
Ahri could go either way. She's interested in humans, not Ionia... Her allegiance (or lack of one) could be decided by the flip of a coin. Is her developing sense of morality enough to convince her to help those in need of her aid, or does she care at all?
Just what I think. Not that it matters, with the lore in shambles (unless you, too, prefer to ignore that the lore was retconned).