non existent base-damage
I'll give you that, but his AD ratios are actually great.
he really doesn't have an identity because he is supposed to be a fighter/assassin but since he has really low base damage, he cant build bruisery to actually be a fighter
You don't need high base damage to build bruisery and be a fighter. The typical bruiser build consists of moderate offensive/defensive itemization. Pantheon is very successful at being a bruiser with
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but his combo pattern also doesnt really allow him to assassinate
What targets are you trying to assassinate by using him? His combo pattern is not the kind that shows up to the enemy bruiser's lane @20-30 minutes in and 1v1 them easy. If you find a lone Marksman, or even if they have their Support with them who has blown their peeling CC, a Spear Shot -> Aegis Stun -> Auto Attack -> Heartseeker Strike deals A LOT of damage to squishy Marksmen. Maybe not 100-0, but when your low cooldown damage spells are back up, it's usually enough to finish the job.
Pantheon is an early-mid game roamer and a late-game team fight disruption bruiser.
You can EASILY assassinate Marksmen, either with a well-timed ult during a team fight and the like. A backline diver, if you will, requiring proficient use of his ultimate.
Pantheon may not have the same play pattern as say Irelia/Jax, when it comes to "burst fighter that can solo your marksman even when teammates are nearby", but Pantheon definitely fills the role of "fighter that disrupts team fights and exploits bad marksman positioning".
His kit is fine as is.
With this next point, I'm not flaunting my rank to show off a big boner or anything. Pantheon may not be Challenger/Master tier level champion, but he can perform well @ Diamond level play. I won my promo series to Diamond with him and is my go-to when I'm top lane in a ranked match @ Diamond level. He may not be an LCS power-level champion, but he's good enough to be played at the highest non-competitive/professional levels of play.