This is my opinion (keep in mind I have absolutely no idea if this is balanced or not)
New Passive: Aspect of War. For every enemy champion hit by one of Pantheon's abilities and for every second that Pantheon is in combat, he gains a stack of Aspect of War. For every stack of Aspect of War, he deals 1% increased damage. This damage is doubled against enemies that are below 50% health. Pantheon slowly loses stacks if he hasn't been in combat for the last 5 seconds.
New Q: Spear Jab/Spear Shot. Pantheon stabs his spear in the target direction, dealing damage to all targets hit. Holding down this ability turns it into a skillshot that deals damage to the first target hit, gaining more range for the longer he channels. Charging this ability counts as being in combat.
New W: Defend. Pantheon raises his shield for 1 second, blocking the first auto attack, turret shot or ability that hits him. If Pantheon blocks an auto attack or a spell, he reflects it back at the champion who hit him, dealing them 50% of the damage he would have taken and stunning them for 0.5 seconds.
New E: Heartseeker Strike. Pantheon dashes to the target enemy unit, dealing damage. This ability can be cast during charging his Q, and the longer his Q has been charged, the longer his leap range. If this ability is cast during charging his Q, he'll deal the damage from his Q to the target he dashes to, on top of the E's already existing damage.
Ult: Grand Skyfall. Pretty much the same, except Pantheon would be able to channel his Q whilst he's falling and every second that he's channeling his ult he'd count as being in combat.