The meta just really doesn't suit him right now. Ranged champs make better use of damage keystones to bully him more than before - preventing him from out-sustaining them in lane so easily as before. Mana issues are much less now with mana flow band. Even last year, other champs could match his sustain in lane - such as fiora spamming q to proc vitals. This year is no different - if anything, champs have just gotten more damage and/or sustain while renekton has essentially stayed the same. Meanwhile, tanks have such high base damage to allow them to farm that they effectively match renekton's damage output while building tanky from item one on. Thus, they are able to absorb more damage while still matching renekton's damage output.
Essentially, he's sandwiched out because he can't trade and out-sustain full damage champs; while from the other end, he can't out-damage or out-tank tanks. The only way he really can win is with effective and repeated early ganks or via massive outplays. Obviously, he still has matchups where he wins regardless of this, but these are few and far between compared to what they are meant to be. And while he can be very scary with the new runes in the right scenario, they either force him to build basically full tank or very heavily in damage at the cost of being too squishy to frontline teamfights.
Suggestions I've made include:
longer scaling stun duration on w
move armor pen to w instead of second proc of e
add armor and mr to renekton ult; maybe something like 20/40/80 armor and 10/20/40 mr
change abilities to scale off TOTAL AD but keep the same ratios
uncap the healing on renekton q
much lower cd spells at max rank; they should be approaching riven levels of spammability
It doesn't make sense for a champ that's designed to become a tanky teamfighter to scale ability damage off BONUS AD instead of total ad; that would be more suited to someone like riven.
Renekton stun locks him in place for awhile AND it doesn't last all that long compared with many other cc.
Due to a sometimes whacky hitbox on e and the long cd, renekton is actually barely more mobile than someone like darius in teamfights - especially if focused with cc.
The armor pen he can get from his empowered e on the dice portion of slice-n-dice is clunky to use. Moving it to w would make it much more usable and it wouldn't make his laning phase any harder for opponents to deal with since you max w last.
If champs like nasus can get percent armor pen and magic pen, more spammable damage via q during ult, and hp added to the resistances their ult provided; then why can't renekton get some resistances added to his ult? After all, they want him to be a tanky teamfighter. Again, this wouldn't be overly oppressive in lane; 20 armor and 10 mr at rank 1, 40 armor and 20 mr at rank 2 of ult. These would only come into play when he ults, so regular trades in the lane would be just like before.