I've always felt that Pantheon's mana problems are kind of the point of balance on him. Yes he basically has a ranged auto-attack every 4 seconds, but if he keeps chucking it whenever he wants he'll be out of mana before he hits 3. I do worry a bit about getting rid of mana on him completely, since in general Riot uses mana as a point of balance, where manaless champions tend to be like Garen, where all of there abilities feel kind of dull, balanced around the fact that there is literally no cost to them.
But on a style perspective, I'd be perfectly fine removing all magic and mana from Panth and focus entirely on the whole part about him being a soldier. Not that Riot is ever going to do that, of course. But I like what you have, if he uses Q he throws his spear as a skill shot and has to use his xiphos. This could lead to a few stat changes. The spear auto-attack could be slower, stronger, but with great reach so it actually feels like a spear, maybe around 200 range. While the xiphos only has 90% AD scaling attacks much quicker and knocks his reach down to 125.
Now for a dive fighter, having his main form of poke, his Q, also make his target grounded sounds neat but may be a bit much.
Way I see it, I do think his general gameplay pattern should remain similar, a form of good poke, and the ability to get right in the middle of a group of enemies and chunk a good portion of health. But then I'm kind of stuck. On the one hand, there usually isn't much for the Pantheon to do after expanding their combo but auto-attack and hope your passives kick in and wait for your Q to go down. This would hopefully get changed. But on the other, I think it is very thematic that Pantheon doesn't have any escapes. It gives him a weakness to exploit and plays on the whole SpartansRakkor do not retreat shtick.