Apparently finding ideas to make skins for Xerath is hard.
So here are some concepts, some from myself, some from the boards:
Dark Star Xerath: there's even a wonderful concept ( https://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/JAgAUXGU-skin-idea-dark-star-xerath?show=flat ) made by SlownD. Dark themes suit xerath particulalry well, due to his personality and story, and shooting deep space energy would fit him like a ring;
The new Dark Cosmic line is also so fitting for Xerath: being so powerful, he could manage the power from both Dark Star, and Cosmic Reaver.
Archlight Xerath: one of the most evil characters in LoL trying to redeem himself? White and gold armor, white particles with symbolism and plummeting dark forces shooting blinding light arcanopulses, or purging them from the skies with celestial arcane barrages;
Harrowing/Haunted Xerath: i mean, his armor is a sarcophagus, just make it creepier with some dark material/monstrous insertions, slap some green/light blue spooky particles, let him unleash his spooky magic while ulting, et voilà, you just found a skin for the next harrowing!
Mecha/pulsefire Xerath: i just need a good mechanical/robotic alternative to Battlecast Xerath. Armor transforms as he casts his spells (to each spell his form) and returns normal after the cast: it'd be so cool to see him transforms while comboing. Orange/yellow particles, and you're good to go.
Void Xerath: this one may be a little bit trickier, but with the organic armor, and tendons or alikes keeping it in place, Xerath should still look quite Void-y while casting purple colored spells!
An ascended Xerath skin would be top of the top, however i don't have any solid concept to go from =/
Battleboss Xerath: like seriously, already playing against xerath feels like a boss battle, where you got to close on him dodging everything he throws at you. he was even the boss in ascension mode, how about that!
Elementalist would be cool too, Q could be a concentrated ray of wind, W water from above, E some kind of lightning, and R could work as a volcano erupting. or you could switch between R and W
also Program, but he really fits well into any tech-y theme, just switch his sarcophagus armor with some metal, slap some cool VFX and there you go
Edit: with Stars Shaper, we went with the idea of a magic lancer xerath: His q could be a spear that stretches as j4's does (maybe he juggles with It as charging animation?) His w can a spear falling from the sky E could be another type of spear, or a morning star. R could be javelins His armor could be like the Lancer zero skins, or made up with weapons and blades, like a moving armory
Also, i'd love if a skin could be made out of his concept art, those black and white particles are really nice:
