Syndra does not need a stronger slow nor a longer stun duration. Making her spheres only stay a second or two if she doesn't hit an enemy champion is not ''not much'', it removes any proactive ball setup or zone control she has with her E, and would result in her becoming unviable if not compensated.
If you meant only getting her passive benifit when she hits an enemy champion (aka base 6 seconds if she doesn't hit an enemy, 8 seconds if she does hit one), then I would be cool with that. Her average ulti damage would go down unless all spheres hit, and would need to follow up if ever Syndra becomes UP, because she's not OP (in terms of power level) at the moment.