Ok, this is the time for drastic measures.
I want a powerful rager who’s a divine caster and can bless her companions before battle and cure their wounds after battle- and go into a killing spree in the middle. I want a possessed warrior in service of a god. Spontaneous casting, may have or not a personalized spell list, may have or not a curse, uses a mystery.
Possibilities include:
-Adjust the prestige class to make it compatible with the Bloodrager and the Unchained rules.
-An archetype for the Warpriest or the Bloodrager.
-An alternate class for the Bloodrager.
-Other?
I want to create a complete manual with customized mysteries, new spells, archetypes and everything. Will you jump aboard? We have cookies.
I think a good rule of thumb while hacking a system is to make the smallest change necessary. Don't create a new class when a new archetype will do. Don't create a new archetype when all you really need is a new feat, etc.
From your description, it would take a lot more work to add rage to the War Priest than to add a touch of the divine to the Bloodrager.
I'd just archetype the Warpriest
trade Sacred Weapon/Armor and one blessing for Rage, give a rage power instead of bonus feat at 6th, 12th, 18th
add ability to Fervor to allow the casting of divine spells in a Rage by using daily use of Fervor (maybe an extra point?)
easy peasy
Skip the PrC entirely.
Be and Oracle with VMC Barabarian.
Make an Oracle Mystery that gives Rage, and replaces Revelations with Rage Powers.
Be a Bloodrager.
Scrape off the word "Arcane" from a Bloodrager and replace it with "Divine". Adjust the spell list as appropriate. Use some appropriate fluff to handle the Divine Focus requirement.
Adding a Mystery means changing the levels the spells come in; you might want to drop some of Bloodrager class features to make space for some revelations.
A prestige class doesn't often play well with PF's extra class features over D&D 3.x unless you're making one for a wizard/cleric (who have few such features). Skip that idea IMO.
If you're looking to make something new then a 6-level caster is easier to do right than a 4-level caster. So a warpriest archetype is the easiest way to make something usable. OTOH the not-LG-paladin is something that more people want, and a riff on that might be more popular. Easy or popular, choose one?
I made a warpriest archetype with rage a while ago. Here it is.
Clerics and warpriests bug me for their alignment restrictions and unlimited spell access. I find spontaneous casters more intriguing.
Barbarians have more or less the same issue: they can't be lawful and I want a character who can be LG.
Rage Prophet is more or less useless if I use a multiclass bloodrager with the Mad Magic feat, but mixing spells from two sources isn't something I like.
A divine casting bloodrager would be the thing I want, but would an archetype be enough? Here's something I just made (please feel free to add your comments).
The chassis is rather easy to do, but there are some issues.
-Which spell list to use? Bloodrager spells as divine spells, as in the Id Rager that just changes them to divine without altering the list? Spell list from a different class? Using the paladin one is not seamless; it requires modifications for different alignments. Just using the cleric one, 1st-4th level, seems too weak. Creating a new spell list? This would require to make it into an alternate class.
-Oracle curse: yes or not? The models of holy ragers in myth, like Samson, Hercules or Roland, made some kind of vow or were haunted, so maybe yes. But should it work like an oracle's curse? If the answer is no, here's another class feature that should be reworked.
-Revelations. One revelation should replace neatly one bloodline power, since the Primalist archetype tells us that one bloodline power is equal to two rage powers. The final revelation is too caster-oriented for a martial class, though. I tried to introduce a capstone that may be the same for everyone, but maybe there's a better solution?