Change Idea
Clerics are one of the few classes that don't have a +5 enhancement option, this makes sense for most clerics given they have access to powerful summons to help carry them through engagements except for the case of Warpriest who cannot summon anything and still have no way of breaking DR due to an overall lack of good breach spells outside specific domains. It may make warpriest more viable overall by making it so they can cast GMW and/or Bless Weapon to get +5 enhancement on their own weapons since they are a far more martially oriented cleric than the others and have decreased DCs as well as no summons.
Warpriest Path Proposed changes:
Magic Weapon/Greater Magic Weapon - Unrestricted when self cast, dropping or trading a weapon with self cast GMW on it removes the buff from the weapon.
Bless Weapon - Warpriest get Enhancement bonus progression for Bless Weapon similar to Paladins.
Since Warpriest doesn't get summons and can't work as high DC save based casters this should make them fit better into the niche they seem to be intended for by making them better martial clerics vs caster clerics. Currently taking Warpriest makes a cleric worse off, here are the "pros" and "cons" broken down in my opinion. This would effectively make them clerics who are good at using a big metal stick to hit things compared to normal clerics which are okay at hitting things with a big metal stick.
Breakdown of Warpriest Currently:
Pros:
+3 CL (at 21st level, so basically still full cleric if you want to be effective and not to have buffs evaporate with 1 dispel)
On paper this looks great but in practice it is basically pointless. Cleric relies heavily on buffs to be even marginally effective and if you don't want those buffs being evaporated by one dispel/breach (1 mords more often) then chances are as a cleric you are already probably going 27/3 or something close to that. I suppose it is nice to be able to get CL 30 with a splash but the small amount of help saving buffs from removal that this gives is not worth the Cons.
Weapon Specialization (at lvl 12 for weapon foci)
I mean nice but this is a 4 fighter splash if you aren't doing some weird build with multiple foci which is on its own probably unviable due to feat starvation
Free Combat Casting Feats
Not as useful as you'd think with -3 DC to All Spells and given you'll be hitting stuff with a weapon.
Double duration on Divine Favor, Divine Power, War Cry
This is nice, it means you have to rebuff less but beyond that bit of wind up being reduced every now and then you are giving up epic spells and all summoning for some time saving.
Spontaneous Cast Aura of Vitality 7th circle.
Saving 1 spellslot that you are going to use anyway if you needed it, if you don't already have +12 Str/Dex/Con from gear and zoo spells (given you take transmutation) you would just slot this, if you do then this is literally useless aside from making your friends hug you in fights making you all a big target for that incoming Hellball that you can't cast. Also probably giving up an extra cast of WoF for this, spontaneous doesn't mean free it just means no preparation requirement.
Divine Smite, but you only get progression if you are Good or Evil domain
Cool, if it wasn't domain locked behind two of the worst domains given you can't even use Gate as warpriest.
Some class synergies which amount to the following if you don't want to lose a ton of CL
Monk - You get whirlwind and Ki Strike 4 (if you invest an epic feat), So an instant attack that is alright and +4 magical on unarmed strikes which is basically worthless.
Pretty worthless if you aren't doing a sacred fist, still not great if you are, also Whirlwind clashes with Divine Smite since they share a cooldown.
Barbarian - Little barb rage (+2 ab, +6 damage) and 1 AC if you don't wear heavy armor.
Arguably a decent small buff not huge but not completely worthless, though odds are that rage doesn't last long with just a splash
Ranger - Ranger TWF (If you wear only clothing or light armor) or Archery, out of combat move speed, tracking, exploration (3 non combat related skills and some feat synergy that is armor restricted or ranged without damage)
Okay if you go Dex and can wear cloth/light or plan to do ranged, but not amazing. Sure you'd get discipline from Ranger if you did it super late which is a nice bonus but overall not really amazing. Non combat abilities should not be considered in balancing mechanical prowess, all 3 of the non combat ranger synergies do not effect a war priests ability to actually fight anything unlike the loss of summons and epic spells which decrease their ability to fight.
Cons:
Lose all forms of summoning.
This makes you less effective in PvE and PvP. PvE because summons make content easier to manage in parties and solo. PvP because summons force players to WoF/Banish/etc to get rid of them, kill them, or take a ton of damage from them potentially if they can't immediately kill the cleric.
Lose ability to take epic spells
MD, EM, PC, and EKD are disabled by the previous Con so lets talk about the other ones which are the damage ones: You basically lose the option to take Hellball, Greater Ruin, or Vampiric Feast. All 3 of these high damage epic spells are pretty nice and if used correctly can be really useful
Lose epic foci commands (-yoink, -teleport, etc)
Meaningless for mechanical balancing given that these are all out of combat perks, though arguably having -teleport to escape combat situations would be nice.
-3 DC to All Spells
Say goodbye to ever being an effective disabler in any way because high saves are basically a cornerstone of PvP. I suppose you could fish for 1s with the few spells you might use but hitting stuff with a big mace will always be better.
In Conclusion
Currently you are basically giving up decent spell casting DCs, Epic Spells, and Summoning for superficial changes and minor buffs to martial abilities. These changes don't really set warpriest apart from cleric by making them a better martial combatant but instead by just making them a massively weaker spellcaster while giving the illusion in writing that they are effective at melee. Currently cleric is one of the weaker classes in PvP engagements, and yeah clerics can do PvE content well enough but a big part of that is thanks to the fact that they can summon much the same as other casters, however warpriest removes that ability to summon.