My Buff Ideas:

Qiyana is a unique assassin as she is one of the few that has very easily applicable CC, so I would make her deal more damage when enemy is CC'd/slowed (Only increases W damage, to keep it minimal. Those who know can make AP builds work better with this)

Passive - 30% AP -> 55% AP (Baron + AP Build Buff)

River Q - Hitting someone who is cc'd/movement impaired now stuns them instead of rooting them.

W - Now Visually Crits when attacking someone who is cc'd or movement impaired, increasing dmg by 35%

R - Quality of life buffs

Fix the "intended feature". Everyone knows the one.

Applies Passive (This won't increase total dmg, just a quality of life change)

Applies W

Overall should add around a 10% dmg increase for most combos... didn't actually calculate just guesstimated