Lux does not need buffs at all, however...
There's no denying Xerath is basically a better version of her currently
However, you do not offset this by buffing a champion who is already pretty decent. The only things buffs do is break her. What you do is try to differentiate her role from his a bit. Lux has pretty decent utility in the form of her W, however it feels very clunky and lackluster because of its delay and the difficulty of using it on an ally. You could simply remove the cast delay on it (meaning the shield applies instantly) and give more clarity to it when using it on an ally.
This doesn't buff her numbers and as a result, end up overtuning her. This gives her role much more versatility with straight up better utility than Xerath with less consistent poke (but more raw power in Q/R). Her utility isn't as good as someone's like Orianna but her damage is higher and her poke and catch potential is stronger.