Lux, Xerath and Ziggs all feel way too similar.
They're all long-ranged siege characters with slows and poke emphasis. Their kits are almost cut out of a mold:
Passive: Some interaction with auto attacks that (theoretically) requires you to get in auto range. Q (or lux E): Long range aoe poke. W: AOE utility E: (or lux Q) hard cc of some sort. R: Near-global damage spell.
All skill shots. They all perform almost exactly the same function, and don't feel terribly different about the way they do it. They're all safe siegers and they all can do massive aoe damage, but fail to seem really different while doing so. Oh sure, Lux crowds out the other two pretty bad right now, but these three champs feel pretty interchangeable.
But riot's reworking Vel'Koz because he "isn't different enough" instead.