Theres kinda only one way to fix Diana.
Remove her assassin playstyle. This is the same exact problem Irelia's rework had, but they decided to go down the assassin route (I hardly call her a fighter anymore, she just bursts). She must become a bruiser, simple as that.
or
should become decent options.
That said, Moonflair spellblade would still be a nice addition in some form, and a proper Armor/AP option that hasn't half it's power put into a stasis active is long overdue.