How about project vs predator? There are already headhunter skins, think about it, that Sci-fi dystopia inhabited by cyborgs could be one of many cities walled off from a wilderness that has reclaimed most of Runeterra, and could be constantly under siege from guerrilla fighters that call those jungles home.
Here's how I picture it,
HT-189; a new resource is discovered in Zaun after digging deep into runeterras crust, a substance that has a wide variety of magical and scientific applications. This substance changes the face of war, whoever controls the most will gain an immense advantage in any battle. Upon learning of this substance Noxus begins a military occupation in Zaun.
Word of this HT-189 and the threat of it falling under Noxian control soon spreads, Demacia pledges it's armies against the growing threat but they are outmatched by the chemically augmented Noxus. Piltover scientists soon discover, using geological notes provided by Ezreal, that the Zaun site was not unique and that many deposits of HT-189 exist throughout the continent. Jarvan IV sends out the legions of Demacia to secure as many of these sites as possible.
Upon investigating this rapid expansion by Demacia, Noxian high command realizes that their conquest is in jeopardy, and Swain sets out to attempt to capture as many of these new outposts as possible. Terrible War engulfs Runeterra, many of it's great cities fall and are assimilated, civilization clings to the few remaining lights in the darkness that has been cast over all they new, the HT-189 excavation sites. Centuries pass and the wilderness reclaims the land, the excavation sites have bloomed into vast walled cities with little contact between them. Giant Mecha guardians and Cybernetic constructs patrol the walls of these cities to ward of the threat of an attack by the scavengers that inhabit these wilds.
Zilean sees these events in a vision and attempts to warn the inhabitants of Runeterra, though its leaders will not heed his warnings, becoming desperate Zilean searches for help from any who will listen. The only way to sway the minds of those with the power to prevent the approaching armageddon is with physical proof, someone must travel into this grim dark future and gather evidence of these atrocities. The task falls to the two people who may be able to accomplish such a task, Ezreal and Ekko. Using the Zero-Drive combined with Zileans Chrono-magic allows them to travel to into the future to new Piltover, in an attempt to learn how these events can be prevented.
So this could tie in the mecha skins and the project skins, the wilderness scavengers would be the headhunter skins, battlecast could even be set up as an early blueprint for the project skins. A Pulsefire Ekko skin could also be released for this, though maybe as a legendary skin instead of ultimate.
