One thing that enrages (pun intended) me to no end, is the fact that you can’t benefit from multiple forms of rage at the same time in 1st party Pathfinder. THE MOST Iconic Barbarian, The Incredible Hulk, literally does this very thing (the angrier he gets, the stronger he gets), so it makes no sense why Pathfinder has such an aversion to it when they love making references to fiction, such as most of the Kaiju being based off of a Godzilla monster, the Brute Vigilante archetype being based off of the Hulk, the Magical Child Vigilante being based off of Magical Girl animes, the Master Chymist prestige class being based off of Dr Jekyll and Mr Hide, and so on.
So basically, I am forced to turn to third party to see if there is any reasonable third party feat or feat tree that allows you to stack rages.
Any help would be appreciated.
If you really want to model escalating rage, there are rage powers which have to be activated during rage: Unexpected strike, surprise accuracy, powerful blow, mighty swing, strength surge etc.. You can even refluff auspicious mark to be more martial.
But if you look for a way to collect several +2 AB & damage bonuses for little cost, I won't support that.
For third party, D&D 3.5 had the Frenzied Berserker which got a rage like ability that says it stacks with rage for some big bonuses and penalties.
Otherwise there is the half-orc teamwork feat Amplified Rage that is sort of like stacking rage. Alternatively unchained barbarian doesn't have the same morale bonuses so potentially you can add on the rage spell or other class features in some way.
Many of the hulk like archetypes from alchemist and shifter etc could probably be combined in some way too but I've heard most of those archetypes are pretty bad. You can also just describe buffs like the mutagen etc to just be added on rage in a role-play sense.
I don't know how to stack different rages, but I know rage, mutagen, enlarge Person and bull's strength/a belt of giant strength all stack for some pretty crazy bonuses ridiculously early.
Take a dwarven frenzied berserker 2/vivisectionist 1. Str18, Dex15. Power Attack, Combat Reflexes. Masterwork giant sticker. Animal fury.
Set against a charge, that's +10 (6d6+16) and two attacks of opportunity at +10 (3d6+16).
On his turn, it's +10/+10 (3d6+16) and +4 (1d4+5).
Assuming an AC16 and at least two targets, that's...122.7 a round? At lvl3? Oh, and sneak attack sometimes or whatever.
I'm sure there are better ways to do that, too. Just...I dunno. How much angrier do you want?
Also, I always thought that was weird. "The angrier he gets, the stronger he gets." So...he just...doesn't have a limit to how angry he can be? No matter how pissed his is in one moment, he could always get a little angrier.
I think that's his real superpower.