I always miss the beginning of these threads, but ah well. Here's hoping what I post gets seen.
Pantheon has gone through a myriad of changes throughout his life in the League, some good, and some bad. Let's take a look at his beginning.
When Pantheon was first released his HSS was bugged, making it take 100% of his AD at all levels. This made him do absolutely obscene damage, but no one besides Pantheon players noticed this. Well, Riot did too, as they fixed the bug a patch or two later. The thing is though, non-pantheon players somehow did NOT notice this bug, and for one reason or another, called him underpowered.
The only lackluster move in his kit at his release was his ult, mainly due to the long channeling time.
Moving along, the patch that fixed the HSS bug also buffed every move in Panth's kit, minus the stun I believe. According to some math I punched in, taking into account his release and post patch stats, led to some pretty incredible numbers.
Assuming you were fighting someone with no armor, and also assuming you had a BF Sword, the post patch HSS was dealing 200~ more damage than release Pantheon. The numbers I have kicking around are 650 release, 850 post. Which is a gigantic leap.
Pantheon was quickly nerfed, making his HSS a passive / weak farming tool. A couple of months went by and I played Pantheon with some pretty good success using Brutalizer, Boots of Swiftness, and then getting Trinity Force (getting sheen first). It was a completely different style of play that still worked for him. The reason this was true was due to his stun. If you were positioned correctly, you could throw a spear, hit someone with a Trinity Force Proc, stun that someone, hit them with the Trinity Force, and then spear again, being sure to make use of the Trinity Force.
That was only possible due to the length of the stun. With that shortened down it made it impossible to achieve the sheen / trinity force procs needed to burst someone down.
Really though, the main kick to his kit that made Pantheon so... eh, not really unviable, just not as attractive as say, Lee Sin, Jarvan, Xin Zhao, was the nerf to the speed that HSS dealt damage.
tl;dr Revert HSS speed decrease, slightly increase stun length, and decrease the channel time on the ult.
P.S. Been playing since the game went into open beta, and I still absolutely love it.
P.P.S. Been playing Pantheon since his release, and I still absolutely love him.
P.P.P.S. Pantheon is still better looking than all the other male champions BOOYAH