In all honesty? Ekko.
Riot's clearly developing him into the "main protagonist" for the Zaun faction, much like Miss Fortune being the main protagonist for Bilgewater, or Lux being the main protagonist for Demacia.
The biggest indicator lies in his narrative "set-up" and his relations to other faction-affiliated champions. At the moment, he currently has lore/narrative connections to:
- Jinx - someone he used to know from his old gang, a friend he once admired (and even developed a crush on) before she went insane.
-Vi - recent lore states that she wasn't a member of Ekko's old gang, but it's possible that both of their gangs collaborated for jobs and various heists in the past. Explaining why he once admired her before she 'sold out' to Piltover.
- Jayce - offered apprenticeship, which Ekko refused.
- Viktor - offered apprenticeship, which Ekko also refused.
- Caitlyn - will inevitably interact with her at some point, thanks to her affiliation with Vi.
- Possibly Warwick - could interact with him at some point, thanks to his heavily-implied affiliations with Jinx.
- Singed - same as above, due to his implied relations with Jinx and Warwick.
- Possibly Twitch - his in-game taunt might be an indicator of some sort of connection( something about Twitch reminding him of a pet rat he used to care for) but it's far more likely it's just a joke that shouldn't be taken seriously.
- Mundo - random encounter as seen in Mundo's short story, and nearly died because of it. Might encounter him again in the future.
- Camille - mentioned under his 'related champions' tag on his universe page for reasons we don't yet know. Though knowing her and her role, she'll inevitably see him as a threat to Piltover's power and will try to assassinate him because of it.
- Janna - also mentioned under his relations tag on the Universe site, for reasons we don't yet know. We do know that he worships her, and I can see her being "summoned" by one of his prayers in another upcoming lore event.
- And now Urgot - His in-game taunts towards Ekko states that he sees "potential" in him, and wants him to become another one of his pawns through the use of his "tests."
tl;dr - every Zaunite and Piltovan champion of noticeable status and power (aka - Urgot, Jayce, Viktor & Camille) has (or will have) some sort of personal 'plan' going for Ekko - and for good reason. The kid is a literal prodigal genius and managed to build a rough time machine prototype by himself. Imagine what he could create when he's older, or with a steady stream of funds and resources. That alone is enough of a reason why said aforementioned champions view him as a potential asset or a threat.
But do I think Ekko has the potential to reshape Zaun for the better? No. Honestly, I don't think any of them do. Instead, I think all of them will play their part in "reshaping" Zaun and Piltover - for the worse.
With the recent direction of the lore (hints of a Noxian invasion, Jhin and Evelyn heading to both cities to assassinate important targets, Urgot and Camille's plans to remove powerful political figures, foreshadowing chemical explosions that kill in the thousands; etc) it seems like Zaun and Piltover are headed for some kind of social uprising disaster that'll completely shake the foundations of both cities, like Gangplank's "death" did for Bilgewater in Burning Tides. I think Ekko will be one of the few champions who will try their best to stop the inevitable cataclysm or 'alleviate' its aftermath, but nothing more than that.