Let me clarify some things about Renekton so that we can better discuss what changes need to be made.
When I write damages, I will use this format
nonfury-fury
1. Slice and Dice is Renekton's highest damaging ability
-Cull does 180-270 damage, while slice&dice does 300-425 total
-Cull has a 0.8-1.2 AD ratio, while slice has 1.5-2.25
-Ruthless predator only does 90-135 damage base, while slice does 425. Almost all of ruthless predator's damage comes from 1.5-2.25 your total AD, so you should never level up ruthless predator first.
2. Slice and Dice Armor Percent Reduction, which is not the same as Percent Pen
25% penetration would happen AFTER flat reduction, while 25% reduction happens BEFORE flat reduction.
For Example:
100 armor vs 50% pen and 50 flat pen leaves 25 armor
100 armor vs 50% reduction and 50 flat pen leaves 0 armor
With no armor/magic resist abilities and a flat health bonus, it is not viable to itemize deeply into damage and cooldown reduction, because you will simply die too fast to even use your CDR.
Renekton works better when you build him tankier. Use defensive boots and combo armor/magic resist with his 600 HP ultimate to take more advantage of effective HP.
Last whisper is a MUST to make renekton's spells do damage late game. It adds considerable flat AD for the ratios, and the percent pen effects all three of his high damaging physical spells.
200 armor vs last whisper Renekton with last Whisper:
200 armor - 25% reduction, 31 flat pen, 46% penetration = 64 armor
Renekton has no auto attack or defensive bonuses, so Frozen Mallet fails in two senses, it doesn't add enough durability for Renekton, and he doesn't benefit from being allowed to continuously auto attack or stay close to enemies. If you wanted to use frozen mallet to help your allies by slowing enemies, you are better off using it on a different character.
Merc Treads, Dorans Blades, Warmogs, Chain Vest, Last Whisper work really well into late game.
Here are what I believe to be the main problems with Renekton.
Fury is impossible to generate late game. Renekton's ultimate should generate fury instantly, like Tryndamere's ultimate. That would allow Renekton to respond to the start of a team fight by immediately having enough fury to do some damage.
Slice and Dice doesn't have enough time between Slice and Dice. I often Slice, start attacking, get stunned, and then can't dice to close distance with the enemy that just walked away. Renekton has no movespeed abilities, he has an extremely long cooldown move ability that can be disrupted half way through with stun spells.
Riven never seems to have the problem of not being able to Q again, because her Q cooldown is much shorter than slice, and it gives her more time between Q's than slice/dice gives Renekton.
All this being said, I think that Slice/Dice actually does INSANE amounts of damage as a movement ability. 425 physical damage base AOE with movement? The damage on Slice/Dice could easily be lowered, pushed into Cull the meek or increase the base damage on Ruthless Predator, and Slice/Dice with lower damage could also have a shorter cooldown.