I've played over 1000 games of Diana on my accounts combined & she I can say without a doubt that she's in desperate need of a rework.
Her kit is outdated and old. She's always off meta, & if she gets buffed to the point where she's in meta (never happened), she'd be way too OP.
Her Splash is outdated. Compared to most of the champions we have now, her Visuals (both splash and ingame) are very outdated. I know its not as bad as some other champions but Diana has sooo much potential for a better splash art.
The current AP items make no sense on her because her playstyle makes no sense. She's classified as a fighter because she has no escape but is built like an AP assassin because of her scalings.
Diana just needs to be defined as a champion & given a small (not even a full) rework. Her E and R are the 2 main abilities that need to be tweaked to make her more fun & healthy as a champion.