Temporary tankiness has been moving toward %DR instead of AR or MR in League design lately - Master Yi replaced his massive AR/MR buff with %DR, Gragas has short term %DR, and Maokai and Alistar are so loved because of their %DR. Why not go in that direction with Renekton? Just honestly curious about your thoughts with that. The other thing is that %DR is multiplicative and calculated in front of armor and magic resistance.
The main thing I would say is not give it that lump of raw stats but instead reinforce tankiness and add some decision making.
The only reason I would argue with your phrasing and methodology is that granting him straight defenses is actually potentially really dangerous. It lets Renekton build raw health with relatively little blowback - a Sunfire Cape and a Warmogs at that point makes him tankier than a Randuin's and Sunfire when he's missing half the health Warmog's grants. Since Warmog's otherwise grants Renekton regen that he needs for longer fights, he's more likely to continue to regenerate health at a rate that would make is so that he'd have these beefier and beefier resistances the lower his massive health pool got.
This is basically the
Poppy problem, but worse in some ways because Armor and MR being what they are, even with the diminishing returns, he'd become an even better tower tank the more the tower hit him. Encourages the already very frustrating diving paradigm that Renekton's early aggression can find him in.
Counter:
Renekton, Butcher of the Sands
Butcher's Heart
Renekton has been hardened by battle and bloodshed. Upon reaching 50 fury, Renekton gains (25 per 1000 maximum health)% Damage Reduction for 3 seconds.
When Renekton falls below half health, he gains twice as much Fury from all sources.
Interesting choices.
Renekton gets Tankier for gaining the Fury, essentially, and holding it. That rage makes him strong, and would now be reflected by him ignoring some of the damage his enemies throw at him. Gives Renekton the option of generating fury for defensive purposes instead of reactive - Q a minion group as a Ryze Q is flying at you to gain the DR you need to survive the assault. You give up the healing an Empowered Q would grant you, but you gain the fury and now have a little room to fight harder or use one of your other empowered abilities.
I think this idea reinforces Renekton's niche as the Tank/Fighter who wants to be in the middle of it, causing damage and destruction and feeling pumped by it.