is in an odd state. In Bronze all the way up to low platinum solo queue, he's a strong top lane pick, capable of dominating. However in high elo and pro's, he's outright never picked. I've heard some people say he should be buffed, the problem is if he got buffed he would be outright OP and perma banned in low-mid elo's. The only times he's been picked in LCS was when he was OP. Not sure how you go about balancing this.
give him a secret passive that makes him deal 9999 true damage to 
and 
whenever he applies a passive stack to them
Juggernauts are interesting, they are seemingly overpowered but in pro play they aren't always picked because those folks know how to kite lol
I think they honestly need to just reduce his Q cast time as he levels up until it reaches instant cast like old Darius. Also, would be cool if he had slightly worse pre-5 stack damage than he currently has, and compensated him by giving him a strong debuff to champs who have 5 stacks like a 20% damage reduction or by reducing their movement and attack speed by a similar percent. Basically make him strong in the wyas he should be, and keeping him weaker pre 5 stack. I don't, though, just a thought. Could also make it so enemies with 3 stacks or more take extra damage in proportion to their bonus movement speed or dash and in proportion to the number of stacks. For example, if vayne has 5 stacks and tumbles, she takes a burst of extra damage. Or having 5 stacks could ground people like cass's E does. Again, to buff him like this it might be good to need his Q base damage just a tad to make his harass weaker, but his all in more viable to punish those who don't properly kite and to actually out duel people once 5 stacks are reached.
