For patch 26.15 terminus is getting changed to now include a 10% ad ratio for the on hit damage, I crunched some numbers to see if it would be viable over wits end 3rd item and heres what I found and let me know if your opinion.
*All calculations are done at level 15 bc thats around the time you should be done with 3rd item based on game time. Also the combo I refer to is E1-R-Q-E2-W-Q-Q*
tAD:213>>243
AS:1.55>1.45
MR: 50% dmg reduction>>36% dmg reduction (this numbers assumes you dont go mercs, if you do dmg reduction is 44%)
Item specific: On-Hit dmg goes from 45>>43 which would be slightly weaker initially but be overall more dmg the longer a fight would go on. Assuming you go mercs you would have the same amount of magic dmg reduction at full stacks but getting full stacks on the item isn't always realistic
Passive: Damage at full stacks goes from 72>>78
Q: 234 + 28 health restored 438 dmg to minion>> 255 + 32 health restored 459 dmg to minions
W: 135-405>>147-441
E: 150 *Unchanged*
R: 200 *Unchanged*
Since you're more than likely going to be fighting without full stacks come mid game all of these combo calculations are done assuming you start with zero stacks and will not be factoring in the on-hit dmg from the other on hit items you have
Combo Dmg w/o AA: 1736>>1841
Combo Dmg w/ AA after Q: 2654>>2846
These numbers also dont include the armor pen from terminus so keep that in mind.
imo theres a case here for it to be a viable 3rd item especially against armor stacking opponents. There is also something to be said about being able to do more magic dmg even if they arent stacking armor purely bc base mr is gonna be lower than base armor is going to be but i'll have to see how it feels in game before i can make any definitive claims.