I'm going to give my view on the points of complexity and theme. I'm not a high ranking player, my only support in this discussion is that I have over 40.1k mastery on Pantheon, but honestly anyone can reach rank 5 in a champion with enough time.
Complexity: Does Pantheon need to be complex? No, I don't mind him being simple I just believe he's a bit too straightforward and people hate that. I think his Q should stay as a targeted ability, but his W should be a skill shot/dash and they should change the active of his E. A skill shot W (short of like a dash in my mind, he propels himself forward in a given direction) would give him two things that would showcase more skillful play; an escape and pursue ability.
As for his E? I just think the passive fits while the active doesn't. If he stays in one spot for long in a team fight he'll end up being crushed. Yes he's a lane bully and falls off mid-late (or just late if he's fed), he's suppose to excel in the lane phase. But honestly how many people even like fighting lane bully champions? I don't like fighting
, before and after his rework. I don't have suggestions for his E active sadly, I just don't think it fits for him.
Theme: I don't think spartan should define him. Take
, before his rework people (in my opinion, so I may be wrong) saw him as "Pirate McPirate" of league, and it seems people see
as "Spartan McSpartan" of league. Personally I'm fine with referencing the spartans (and a lesser extent 300, though I prefer just spartan) but I don't think that should define him. He's the paragon of his people, a tribe built around providing powerful warriors as mercenaries. Riot made Gangplank into, "In a city of people you don't want to meet in a dark alley, he's the guy they don't want to meet in a dark alley."
Look at Pantheon's ult, he comes down onto the battlefield like a meteor. Can you imagine seeing that in the middle of battle? Someone just drops down from the sky and proceeds to decimate the enemy. You should beware the skies if you war with Targon, beware the skies if you face the paragon of Rakkor. He's a damn master of the battlefield.
Personally I think he could use a Gangplank level rework. I'd like him to be less of a lane bully so he can scale better throughout the game and experiment with more build paths rather than capping on his early game power.