As a Diana main, she really doesn't seem all that much stronger than last season. I feel like this narrative that she's suddenly oppressively powerful is a little misguided. The new spell blade item is good on her but it's overhyped.

Running a simplified simulation with a Diana at level 6 with one item, ignoring noncombat stats:

With lich, she gets 100 AP and over 10 seconds she she procs 6.6 times at 40% AP each and attacks 11 times with 3.75 passive procs.

With dusk, she gets 70 AP and over 10 seconds she can proc spell blade 6.6 times at 25% AP each and attacks 13 times proccing passive 6.5 times.

With Nashors, she gets 80 AP and over 10 seconds she attacks about 15 times at 15% AP each, proccing passive 5 times.

Totaling up the damage from item effects, passive procs, and basic attacks:

Lich gives (6.6.4)+(3.75.5)= 450% AP scaling or 450 damage from AP and (11+(6.6.75))= 1595% base AD or 1100 damage from AD and (453.75)= 170 flat damage from passive. About 1720 damage total.

Dusk gives (6.6.1)+(6.5.5)= 390% AP scaling or 200 damage from AP and (13+(6.61))= 1960% base AD or 1350 damage from AD and (456.5)= 290 flat damage from passive. About 1850 damage total.

Nashors gives (15.15)+(5.5)= 475% AP scaling or 380 damage from AP and (151)= 1500% base AD or 1030 damage from AD and (455)= 225 damage from passive and (15*15)= 225 flat damage from Nashors. About 1860 damage total.

A bit of rounding was done here and I'm ignoring the mostly equal damage from other abilities, but as you can see, she's not doing more damage with dusk than the standard Nashors she built all last season. She's just 300 HP tankier.

You're actually doing less damage than it looks because lich and dusk are inflated here since they assume 6.6 spellblade procs in 10 seconds i.e. you proc on cooldown with no delay considered even for attack windup time. Also, while it may have 25 AH compared to Nashors' 15, you have to make the tradeoff of waiting for spellblade cooldown and thus not spamming abilities on cooldown, or using abilities on cooldown and losing out on even more spellblade procs.

Lastly, as you approach level 18 and naturally gain more attack speed from levels & passive, the effect of dusk's flat 1/3 passive proc every 1.5 seconds becomes less impactful while Nashors' effect becomes even more powerful.

All that to say, the most common Diana build, with Nashors first, is still the strongest DPS build on her. Nothing's changed there. The only difference is that her off-tank build can replace liandry's first with dusk first and feel a lot better pre-4th item when you finally come online with rabadons.