Te true answer to this lies as such: Make a scaling healing reduction/theft item.
For isntance, those of you who played wow PROBABLY know about the Death Knight ability (if they still have it) Necrotic Strike. It literally applies a debuff that steals healing from the target until the 'stacks' or 'amount' was used up.
For instance you are Cait with 50% crit and 100 AD (so 200 point crit). the enemy is a swain. cait has w/e this item is called and steals 15% of the attacks damage as healing.
Swain is healing for 75 hp/s
cait autos twice over 2 seconds (1.0 AS)
cait crit once and normaled once for a total of 300 dmg.
cait stuck a debuff on swain that sticks anti-healing stacks for 30(200 crit) + 15 (100normal) = 45 damage, healing her.
swain WOULD have healed for 150
now heals for 115 over that 2 seconds.
Now my math is pretty bad but that is a decent reduction, is completely dependant on damage/as/CDR(if the item uses abilities). It works as an antihealing item, lifesteal item and depending ont he stats put on it could be useful for ADMelees, ADCasters OR tanks.
Personally i find that both tanks and AD need a mana regen itemization that isnt designed for casters (Unholygrail/Manamune) or offers something other than jsut mana regen and damage.
----name----
55 AD
10%cdr
+(25/50%) mp/5
UNIQUE passive: Traumatic Sadism (roflmao)
25% of damage done is applied as a stacking debuff. Heals received by the target give 50% of the healing received to the owner until all stacks are consumed. stacks decay rapidly starting 2 seconds after the last stack was added (so basically if you change targets.) Is not applied to Multi-hit abilities or items. (treated like vaynes silver bolts + runaan's hurricane)
vamp scepter(800)+Brutalizer(1337)+forbidden idol (600g)+ 263g=3000 gold
Alternatively would be cool to see that healing that the 'caitlyn' would get be turned to both mana and HP at a 50/50 ration. so if you steal 100 healing you only get 50 hp, but also get 50 mana. Could replace the brutalizer with chainvest, giantsbelt, negatron cloak or a pickaxe (would increase the gold cost to combine and change the AD to Armor/hp/MR depending on the item.
Also would be cool if you could instead enchant items... for instance enchant BOTRK to cause 15% of the healing you received as anti-healing on them. would be the same lifesteal IF the enemy was being healed.