I've come to appreciate his passive after a while. It's primarily an early game passive, which allows you to win most lvl. 1-2-3 (mostly 1) duels since at those levels the damage is interesting and the slow quite helpful to finish them off. Afterwards it does much less, but that's because of RITO. It used to do 5 + (lv.) damage instead o 3 + (lv.) and could stack to up to 5 times instead of three (for 400 ~ magic damage + 35 % slow at full stacks late game instead of todays 180 ~ + 21 %). Anyways, I'd gladly give it away for any form of CC stronger than slows xD The idea of rooting people at 3 stacks expressed by Zazusha is interesting indeed.
If it depended on me, I'd trade E's active (but not the passive movement speed, which is iconic and nice), the "global"ness of his ult (giving it Pantheon's ult range, perhaps) and maybe his passive for some form of relevant CC and increased dueling potential (maybe in the form of damage). Should his Critplank identity preserved, I'd like it to receive some more consistency (perhaps in the form of passive crit. chance, increased Q crit. chance or the likes).