As I main Lux, I may have some bias towards what I'll discuss but nonetheless here's my input.
I honestly don't believe Lux is deserving of a nerf. Her kit functions relatively simply and thus doesn't require a large amount of mechanical skill to utilise effectively. This is also why her win-rate tends to hover around the 50-51% mark; because she's a safe pick when played in the right hands.
Sure, she's popular now, but that can be attributed to the release of her Ultimate Skin. Her wave-clear early-mid is actually quite subpar and that's because she's reliant early-mid on proccing her passive in order to actually kill anything (since E's damage actually cannot one shot the wave), which, as requiring an auto attack to proc takes much longer to clear when compared to the likes of Viktor. In addition, wave-clearing with E, though her most effective option is quite mana costly and is not as 'free' as many may describe.
Her ultimate offers nothing but vision and damage in a singular, thin and well-telegraphed line and her kit is based around it. It procs and then reapplies her passive and provides the brunt of her damage; her q and e functioning more-so as softeners and appliers of her passive than anything.
Her w is hardly as great as it once was, and functions really only as a shield to counteract poke upon engaging.
Late game, Lux functions as a burst mage - because she is one. Her landing her q on you is reason enough to warrant a kill. Not to mention, the only people she will ever burst - even late game - are the ADC and the squishy midlane mage/assassin.
The counterplay to lategame Lux is literally, watch for her q and jump on her face. In teamfights, she'll throw her e for damage and w for sustain out waiting for a possible pick on the ADC who is close enough and not paying enough attention to be snared by her q. The only time she will ultimate during a teamfight is for a 3-5 man Ultimate (which MUST include at least the adc or squishy mid-laner or otherwise tanks/bruisers laugh at her) or if she has a pick on a fed/squishy enemy champion. That in itself is more of a positioning issue for your team than anything.
Though she has the range advantage most of the time, Lux cannot deal with anyone up in her face. Any diver or form of cc that enables your team to collapse on her will pop her like a balloon. Trust me. She's immobile and reliant on her skills. If you dodge her q and are in range to attack her, she will die.
Finally, itemisation against her is incredibly efficient anyway.
Completely negates her combo if you get caught out by a Q.
Will allow you to live through her burst.
Did you Q me? Nope.
Not to mention
or
which turn any engage in your favour as she loses 1 out of the 3 of her damaging abilities.
TLDR: She functions as an immobile burst mage with small areas of utility; she is not overtuned.
However, I'm up for discussion if there's specifically anything that genuinely bothers you that doesn't fall in line with her "omg she burst tf out of me" identity.