His global presence through mandropping seems pretty iconic, but beyond that a lot of his kit seems bare bones from a thematic standpoint
I don't play Pantheon a lot so I don't actually know what about him panth players like.
His ult is very iconic to him, so that will probably be preserved over everything else.
His passive has a nice level of interaction with both outplay for Panth and counterplay for his foes, so I see no reason to touch that.
His Q will probably be made into a skillshot like Ryze.
His E will most likely be changed completely, since it's very low interaction and not very exciting for anyone involved.
His W might be changed, but I personally think it should be kept a point and click stun as a direct counter to mobility.
He should still want to be a baker. 
Ok... I'll bite. I play a lot of Panth and though I'm not diamond or anything I like to think I understand him pretty well. He's pretty simple, after all.
Pantheon's theme fits him well. Not "generic spartan", but "dedicated warrior". Look at him. He's not a huge, burly mofo like Darius and Garen. He's not an unkillable berzerker like Olaf and Tryndamere. He's not channeling a god like Illaoi, or a viod monster like Rek'sai. He's fairly sleek and slender compared to a lot of his beefy fighter rivals. This is key to understanding Panth, I think. Warning... lore incoming.
Pantheon is the guy who has worked his ASS off to become a warrior. He wanted to be a BAKER for god's sake... but his people fight so he learned to fight. He's a student of the ART of war. Not a dumb bruiser and not a man who can rely on his innate size or mystical advantages to win. To play Panth... you gotta think. Oh sure, he's brutal in lane and thrives on aggression. He's a warrior, after all. But every minute that passes in game is time you could be winning ticking away. Pantheon needs to come at you hard and strong, and he needs to get a lead and make the most of it! He's a team player, despite his macho demeanor... and using panth well means using your ult to aid teammates and crush the enemy together because he literally can NOT carry a game unless he's MASSIVELY ahead. That's what Panth is to me... that combination of aggressive "Ooh-Rah!" macho aggression and strategy. If he wastes his early game, he's boned. Like... reverse-nasus or something. He falls off too hard to sit around. Always pushing foward, always making aggressive plays.
He's mechanically simple, and I like that (Not every champ needs a kit only Faker can play well and 3 passives), but he requires you to MAKE MOVES early. If you're Pantheon and you haven't gotten a kill or at least pushed the enemy from lane within the first 8 minutes... you've failed. If you're jungling pantheon, you need to gank EVERY time your ult is up. Even if you stomp your enemy, you NEED to keep momentum, because you fall off. You need to be feeding a teammate or making your power count via objectives and the like. 15/3 Pantheon can lose and lose hard if he lets the game go too long.
I don't know if I'm explaining it well, but that is what I feel Pantheon is about. He wasn't born a warrior... he MADE himself a warrior. And to win he has to be a freaking warrior for every moment he's on the rift. No idle farm lanes. No casual dances around mid tossing skillshots and backing off. Go. Win. Push. Dive. Kill. He needs to be focused and always pushing towards that win. His eye is on that nexus... the kills are just a means to get there. If they rework Pantheon... and I really hope they don't anytime soon since I have NO faith in riot's reworks, I hope they keep him as the early-dominator he is. Oppressive early but unable to play selfishly without taking on a large risk. Able to get a lead and then snowball every lane via dives and ganks with his ult... and unlike a lot of top laners who just kinda fap around top splitpushing... he HAS to help his team. The teleport top meta kinda stole Panth's thing and made it mandatory for top laners, but it doesn't diminish his skill at it. Kill the top, dunk mid and get them a kill. B or walk bot and get Vayne a kill. Teleport bac top to stop the inevitable push. He's all over the map, looking for places to make plays or give his team an edge. Strategy with a simple kit. He's not fancy... he does what works. He doesn't care if you want to call him "unskilled". Victory is his goal, not praise. Not showing off. Not racking up kills. He is trying to win, and his simple kit has all the things he needs to do so... if you play it right. I hope they never rework him.
There is nothing to rework spellswise. Individually, the spells may look meh, but together they form the iconic Spartan. Much of the rework would be in a vein closer to Ryze where targeted spells might become skillshots in exchange for versatility and counterplay. I understand the need to increase counterplay, but as long as Talon's kit exists as is, Pantheon's low counterplay should remain because Pantheon is honestly healthier than Talon and has a decent amount of strategic counterplay even if the tactical counterplay is lacking.
It's also hard to justify changing any of his spells to something completely different because they are so clear in their intention and effects unlike the bullshit newer champions come up with. Q is a nuke. passive blocks autos and has a counter that is gained via spells and attacks. W is a gapclosing stun that instantly gives him a block. E is an aoe conal that hurts. R is mandrop with a giant red circle.
As opposed to:
WHY ARE YOU GETTING OUT OF KINDRED'S ULT?!
Uh, because it keeps her from dying? We need to get out of it so I can kill her.
...
I think it would be cool if when Pantheon mandrops, the force of the impact creates a giant crater like a meteor strike (think J4 ult but far larger). I would also like for his to keep his HSS, I think it would be awesome if they actually made it's hitbox more like a graves auto, making it take more skill to use, making it easier to evade, but also giving it a little more range. I'm pretty openminded about Panth, hope they do a good job once they get to it.
Definitely keep his man-drop and his passive(Especially from towers!). Very fun to use and see used. Q and E should probably go because of their point and click 'This happens now' nature, but the crit passive on his E I think should be kept. W is up on the wall. On one end it's a point and click gap closing stun, on the other end it's a LOT of his power. Maybe Riot will come up with something clever for it, but whatever it is keep it tied to his passive.
Speaking from a purely mechanical stand point, I think they should definitely keep the mandrop. It's the most integral part of his kit. I think there should be more complex mechanics centering around his shield and spear, since that's what I think of when I think of Pantheon. Perhaps, there could be a stance mechanic that changes how he functions as a character?
Keep his whole kit. He only needs 2 nerfs in my opinion:
Nerf his spears mana cost / cooldown / damage. Heck, even 2 of the 3. Make him a bit less oppressive in laning phase.
Make his 'block' not work on auto attack modifiers. E.g. when I tried to flip a Pantheon out of diving my ADC using Volibear's Q, and he blocked the hard CC. Not cool. Block auto-attack damage, sure. He shouldn't block CC though. It's a limited case, but seems unfair.
Nothing is wrong with his kit. Taric, Nunu, Urgot, Yorick, vlad, and maybe even Olaf, Galio possibly need a rework. But panth is still relevant.
I think heartseeker is another skill that should stay, the high damage cone channle. If counterplay is an issue it can be modifyed, like increasing the total damage and duration a bit, or giving him some stacking up damage whenever he deals damage to an opponent, so a single heartseeker won't deal much damage, but increase the follow up damage quite a bit and a second heartseeker would get devastateing(think of old mf W+ult)
For a generall direction, keep him as a abillitx based fighter, dealing most damage with skills on low cd while lacking good backline acces unless you land a great mandrop, but compensateing for that with solid tankiness. The point n click stun to deal with nimble targets also is rather important for me and often enough the reason i pick him. Getting a save lockdown v the likes of LB or lucian is rather huge these days.
What i like thematically is that he's a pure warrior, no magic ir stuff(unless you count the ult), just a man, his shield and a spear and thats it, which is why i dislike the new lore. Now he's 'Imbued with inhuman power', whie befor he jsut was a badass warrior who if asked "How comes you're so powerfull, are you a god or something?", he woul just answer "No, i'm just that good'. I also like how he's an artisan of war and that shows in how he plays. He searches the fight at his terms and plays rather tactical to motigate his weaknesses, especially with the ult, but also by playing around his cooldowns and passive and by how imprtant his positioning is in a fight due to heartseeker locking you down but dealing vast damage. Sadly 1v1 early on he's so overpowering that playing tactically isn't required in lane, though there is a big fifference between a good and a bad panteon in teamfights.
I'd say his passive and his ultimate are going to be preserved. They both have places in the game, I just think they could be more interesting. I think his passive should stack outside of Wing someone at 3 charges, and his ultimate could be much more supportive.
His Q would most likely get made into a skill shot. His W is most likely going to get axed completely, and his E is probably going to be heavily reworked too, but I think the "crit execute" mechanic is going to stay.
Most likely his ult would be kept, but his other basic abilities could use some changing around. Hopefully not another "his next 3 AAs becomes empowered" bs.
I'm an "outsider" as I don't play Panth but from what I know about him I'd preserve his ultimate and the block mechanic.
I'd keep his passive and ult. The rest if his kit is kinda forgettable.
