I'm writing this out of an inspirational spree (and boredom).
I haven't read Pantheon's lore in a while, hence, I don't know if his lore is tightly tied to his Spear&Shield Combo.
This is an attempt in giving pantheon a multi purpose tool on his Q but keeping him kind of predictable once his turn has passed.
Passive - Artistry of War
Artistry of War
At 12 Stacks Pantheon can switch weapons into (weapons load out is showed as kindred's or rengar passives)
Spear and Shield - Mild increase in range moderate increase in damage
Sword and Shield - Short range, defensive bonuses + 5% tenacity
Labrys - Longer range than sword, shorter than spear, moderate increase in damage. decrese in defenses
Upon switching weapons, pantheon gains a burst of movement speed and a shield Scaling X to (X)
Q - Weaponmaster
Passive - Killing any unit grants Pantheon a stack of "Artistry of War" killing an enemy champions grants 6 stacks
Q Spear - Spear Shot: Pantheon throws his spear dealing attack danage
Q Sword and Shield - Tactical Wound: Pantheon lunges forward with his shield at the ready can reactivate the Q to execute a stun in front of him or wait the end of the dash that will make him thrust his sword forward dealing damage in a line and slowing all units hit for 45%
Q Labrys - Pantheon swings his double edged axe in 2 wide and slow cones dealing each X to X damage and healing him for 5% per champion hit. Pantheon can walk during the ability execution
W - Phalanx!
Pantheon completely blocks the next tower shot/basic attack or ability that hits him (Only champion or epic monsters can trigger this abiliy).
If pantheon successfully blocks, all allies in range obtain a shield
E Spoils of War - Pantheon's next basic attack is empowered and upon successfully killinh an unit resets the passive bonuses on switch.
R Grand Skyfall -stays unchanged from the official one.
NEW R Aegis Warrior Pantheon leaps into the air and smashes the ground at a distance X slowing all enemies hit in a radius for the next 4 Seconds pantheon can switch weapons without however gaining any of the bonuses on switch.
WHERE DOES HE KEEP ALL THIS STUFF?
Well I remember that he is an incarnation of a god of war, i'd say he can freely and efforthlessly craft weapon out of thin air appearing in his hands in bursts of light, should I mention though that the switch to another weapon should cost him 1 second of uninterupptable animation.
Honestly I know this maybe result in something overpowered. and I have no hope to be answered by actual rioters on this however, I really like to throw in ideas for one of my favourite champions. Have a good day Y'all :D