Now look, Pantheon is always going to be my favorite, outdated model and 'walking parody' alike. I do welcome changes and fleshing out to the story, though and personality is one thing I'd like to be touched on. The only 'connection' thing he got so far is 'He's Leona's childhood friend'. Pantheon certainly has a personality, he's serious, battle-hungry, and dedicated about the art of war. He's out to seriously show you how you lob your spear into the heart of the enemy. He wants to represent his tribe the best he can, how was that different from Garen and Darius? You can say that Garen is all protective and stuffs like that because it's what Demacia represents, while as of Darius, he's calm, collected, yet brutal and has a distaste on weaklings, the stereotype of most Noxians.
Pantheon is like that, he represents his people at Rakkor. Leona is also serene and protective because she represents the 'good' side of Solari, which coincidences with her personality. Diana is ruthless and vengeful because she represents the lost and betrayed tribe of Lunari.
Now the problem with Pantheon, which I'm going to admit, is that... NOTHING differentiates Pantheon with 'Rakkor Warrior #43241'. We could even say that Pantheon is just another Rakkor warrior, only the best there is in combat, but it's like saying the Rakkor tribe is a tribe full of Pantheon copies. It takes a JOKE to make him look a little unique, yes, the baker joke. While I appreciate the joke and find it the funniest joke in League, I think it's not a good way to express a distinct trait.
My suggestion is that not only Pantheon gets more in touch with the Leona-Diana rivalry (I dunno how it'll go, though, anything goes, although I kinda think that Pantheon-Leona is more applicable here because Diana was raised in the Solari sect since birth, so they'd be like totally strangers), if there's another champion migrating to Mount Targon (kind of how like Nasus, Renekton and Skarner changed into Shurima natives after Shurima retcon), maybe linking up with Pantheon could be the way to go or something.
Just my two cents. If Pantheon, with all his flaws story-wise and model-wise impresses me like this, I'm sure the new story modifier for him will be great and he still attains the place as my favorite. I don't think you even need to completely overhaul his skill set, after all, when Sivir got modified, she still retains her original skill set.