They should introduce a wider variety of stat debuffs and maybe stat-saps on Supports (Critical Strike Damage reduction?).
I don't think we have any kind of confuse debuff yet. Maybe something that causes enemies to walk in the opposite direction of what they click or to attack random targets instead of what they click on. Also, maybe a confuse effect that causes them to be able to damage their allies with a % of their AoE damage for a while (like only 10% or so) which would force Mages to be more careful in their spell placements. This effect wouldn't work on allies beneath X% Health, though. Another confuse-esque effect (or maybe Enrage or something) could be hiding an enemy's Health bar from them and their allies for a duration.
As an alternative to Grievous Wounds, maybe something that damages a champion every time they heal or shield an ally or maybe a selective silence that prevents them from using active heals and shields (or just selective silences for various things in general).
Maybe a debuff that prevents an enemy from slowing your allies (like a Holy Fire theme or so). Could also be a buff on an ally.
A debuff that taunts minions into attacking a target.
A weird concept I had was for an Earth-bind debuff tied to maybe a Poppy or Vayne displacement that would ground enemies and force them to walk alongside walls or terrain (if the spell pushed them into a wall). Other terrain stuff could be setting brush on fire or freezing the river.
Another cool debuff could force enemies to automatically dash a little distance towards a specific location (your base?) once or every few seconds.