Here is my throw at a rework for Skarner. Even if Skarner is not in need for one, tell me what you think of these ideas.
-No longer grants passive movement speed. Instead it changes 'Crystaline Exoskeleton' 20 movement speed reduction to a 70-140 bonus movement speed.
-Damage changed to a base damage +30% total AD physical damage.
-Hitting an enemy now grants Skarner a stack of Crystal Slash, stacking up to four stacks. (one stack per ability cast at max) Each stack increases the damage dealt by Crystal slash by 50%. At four stacks Additionaly deals 5% of the enemies max health as magic damage.
-While 'Crystaline Exoskeleton' lasts, knocks all enemies 100 units away from Skarner, all but the last target skarner auto-attacked. This knockback does not disable anything and is not Yasuo compitable. It merely moves enemies.
-Cooldown reduction remains the same.
-Shield duration lowered to four seconds.
-While the shield is active, Skarner slows himself for 20 movement speed. Skarner can reactivate this ability after two seconds to end it early. (imagine bigger, heavier crystal layer on Skarner)
-While the shield lasts, on-hit Skarner deals 20/30/40/50/60/70 bonus magic damage and restores his shield for the same amount if it is damaged. This cannot exceed the shields normal maximum. Versus champions, also increases the duration of the shield by one second on hit. Cooldown starts after the shield has been breached.
-Cooldown lowered to 6/5.5/5/4.5/4 seconds
-Changed to an auto attack enpowering effect. For his next attack, gains 250 bonus attack range, and fires a crystal blast from his tail. This attack is still classified as melee. Applies crystal venom for 1.5 seconds, slowing his target by 40% and dealing damage every half second.
-Movement speed from impaling reworked. Now gains the 70-120 bonus movement speed if Impale is cast while 'Crystaline Exoskeleton' has been active for atleast two seconds, resulting in him shattering his shield and gaining the bonus movement speed for the duration of Impale.
My thoughts: I've never liked it that Skarner had so much chasing potential while being tagged as a juggernaut. With the changes to Crystaline Exoskeleton Skarner is suposed to crawl himself in the middle of the fight, then activate it and attacking enemies to become unkillable, shoving away low priority targets with Crystal Slash and potentialy tagging his high priority target with Impales new passive, and then, Impaling them.
I've also always liked Skarner's synergy with attack speed, so I further emphasized that aspect. Also unlike Nasus for example, Skarner does not nuke his target when he reaches them but shreds them appart slowly but surely. About that surely part, I think that Skarner might need something more to stick to his target when he is on top of them, which could be his third ability but I have no clue.