Buff Diana the right way!
Diana base AD now: 53 + 3 per level
Comparison:
57 + 3 per level
61 + 3.5 per level
61 + 3.2 per level
60 + 3 per level
58.8 + 3.9 per level (WTF!)
61 + 3.6 per level
these are the "wtf" ones
heres all the other ap/ support champions with higher base AD than Diana

Note I did not even include 
But the most laughable thing is Diana's base mana and mana growth Base mana: 297 + 40 LO FUCKING L
The only AP champion lower than that is
However at level 2 they become even and then at level 18 Ekko has 1130 mana and Diana has 977.
Diana has the lowest mana in the game as far as any AP champion is concerned. This is fucking pathetic.
Ahri for example has 334 with a growth of 50.
But wait mana overall isnt complete without base mana regen which is the exact same for every mage champion and then Diana. Is Diana a mage?
Diana: 6 + 0.8
These are clear evidence of why Diana is in the current state that she is. Base AD and mana is the only buff Diana needs and any rework that you do to her is just avoid this clear problem.
would be pretty amazing on her. I don't know the optimal route to get those items though - like if you start roa, you will be weak early game damage-wise; if you start the other items, you don't have the tankiness to make use of your aa and spell vamp if focused. But if you can reach something like those 4 items, you should be reasonably tanky and have a ton of dps. I guess you could also do deathcap or something in place of roa and rely on w shield for tankiness too maybe.
then pick items I feel like would be good from there. If I'm ahead I get a guinsoos because I don't need to be tanky aside from RoA.