4 Suggestions, 3 buffs and 1 heavy nerf, to make Renekton's Ult better thematically and just better overall.
1: 5 Fury per Second replaced with Fury each time he deals damage to an Enemy Champion with his Ultimate AoE
Just like the last changes he received, this focuses on highlighting his more aggressive and assertive playstyle and rewards him for taking a proactive and dominating approach in teamfights.
2: AoE Damage reduced but made into Physical Damage and AP scaling replaced with Total HP
Less damage overall until a certain Health bonus, which he already reaches. This will weaken his early game use of the Ultimate a tad but will heavily buff his late game teamfighting which is already kinda subpar.
3: Increases his Passive to 75/100/125% increased Fury from all sources when under 50% health for the Duration
Gives his Passive some use and makes Renekton want to use his Ultimate more often when at low health to turn the tides of battle.
4: Duration reduced to 10 seconds from 15 seconds
Lower Duration mainly to allow the former 3 buffs to exist without warping his power. Specifically made to give him a reason to use the Ultimate to turn the tides of battle in a shorter window of time allowing for more plays on Renekton's part but also allowing the enemy team to counterplay and punish a Renekton that uses his Ultimate poorly.